Identify correct chuck to fit this back plate. Van Norman 7 1/2 dividing head

[Blue Rabit]

This little backing plate came on the 7 1/2" Van Norman dividing head that I picked up a while back. I think it may be original but I do not have any history on my tool. It has no markings on it at all.
It does appear to visually match the profile of the one in the technical drawing of the 7 1/2 dividing head that is on the vintage machinery site.
I am hoping that someone here recognizes the mounting dimensions and can point me towards a mfg that I can then set up an auction search for.
I have been searching without success to identify the chuck that would have mated to it.
I have been digging through the old catalogues with dimensions for cushman, buck, skinner, and union. I have also reviewed documentation for newer products such as pratt-burnerd and bison.
So far both Cushman and buck are closest but each only has some correct dimensions.
The spindle threads are a 1 7/8-12, not a standard spindle thread so finding a used chuck/backing plate combination is unlikely. I know that it is fairly straightforward to modify the plate that I have to fit another chuck, but that would still leave the mystery.

Dimensions:
OD: 4.25"
Bolt pattern: 3.625" bolt circle, 5/16-24 threaded hole, qty 3 evenly spaced
raised centering boss: 3.125" dia, .11" height

I assume that it should have a 4" chuck on it.

At some point in it's life, someone used a wrench to remove it from the spindle. Other than some scarring from that (which will clean up with a bit of stone work) it is in excellent condition.
The image with colored areas is one I used asking some questions on another site but does show my dividing head. in it you can see the original back plate behind the chuck. There was a very poorly made adapter in between that introduced a lot of runout.

 

[johnmontrose]

You are probably way ahead of me here, but if the holes in the backplate are threaded, it means the chuck must be front mounted. That would narrow down your search a little.

The 'raised centre boss' is often called a 'register' so looking for a front mounted chuck with a 3 1/8" register might find something.

 

[Blue Rabit]

Yes, the holes are threaded, 5/16-24.
I have been operating under the assumption that the chuck that once mated to this is no longer in production and that any documentation would be in the form of non searchable pdfs.

 

[sfriedberg]

Blue Rabit, if you have a lathe you can make a backplate to fit any reasonably sized chuck.

If you are limited to your existing backplate, you need to find a 4" chuck with front-mounting holes with the same bolt hole circle diameter as your backplate or modify a 4" chuck by drilling and counter-boring new front-mounting holes on the proper bolt hole circle. Ideally, you'd find a chuck that locates on the existing backplate register. But if you find a chuck that otherwise fits but has too small a recess, you can use a lathe to turn down the register on the backplate to fit. If you find a chuck that otherwise fits but has too large a recess, you can use a lathe to make a snug-fitting ring to take up the slack or face away some of the backplate, drill and tap four radial holes near the rear edge of the chuck body, and turn it into an "Adjust-Tru" type of mount with radial screws bearing on the extended register. With Adjust-Tru mounting, you want slightly oversized front-mounting holes so you can use the radial screws to dial in the chuck on the spindle centerline.

Lots of options if you don't restrict yourself to trying to find some specific ancient (and unknown) model of chuck.

 

[Blue Rabit]

Blue Rabit, if you have a lathe you can make a backplate to fit any reasonably sized chuck.

If you are limited to your existing backplate, you need to find a 4" chuck with front-mounting holes with the same bolt hole circle diameter as your backplate or modify a 4" chuck by drilling and counter-boring new front-mounting holes on the proper bolt hole circle. Ideally, you'd find a chuck that locates on the existing backplate register. But if you find a chuck that otherwise fits but has too small a recess, you can use a lathe to turn down the register on the backplate to fit. If you find a chuck that otherwise fits but has too large a recess, you can use a lathe to make a snug-fitting ring to take up the slack or face away some of the backplate, drill and tap four radial holes near the rear edge of the chuck body, and turn it into an "Adjust-Tru" type of mount with radial screws bearing on the extended register. With Adjust-Tru mounting, you want slightly oversized front-mounting holes so you can use the radial screws to dial in the chuck on the spindle centerline.

Lots of options if you don't restrict yourself to trying to find some specific ancient (and unknown) model of chuck.

I will likely end up making an adapter to go between my backing plate and another chuck. I dont want to modify this backing plate just yet in case i do eventually figure out the correct chuck for it. At this point it is more about solving the mystery than actually getting the chuck that was once on it. I am surprised that it doesnt match any of the big players of that era that I have already ruled out (cushman, buck, union, skinner). given that all the measurements are clearly sae, it is unlikely that it is not a us made chuck.

 

[ReedReed]

This little backing plate came on the 7 1/2" Van Norman dividing head that I picked up a while back. I think it may be original but I do not have any history on my tool. It has no markings on it at all.
It does appear to visually match the profile of the one in the technical drawing of the 7 1/2 dividing head that is on the vintage machinery site.
I am hoping that someone here recognizes the mounting dimensions and can point me towards a mfg that I can then set up an auction search for.
I have been searching without success to identify the chuck that would have mated to it.
I have been digging through the old catalogues with dimensions for cushman, buck, skinner, and union. I have also reviewed documentation for newer products such as pratt-burnerd and bison.
So far both Cushman and buck are closest but each only has some correct dimensions.
The spindle threads are a 1 7/8-12, not a standard spindle thread so finding a used chuck/backing plate combination is unlikely. I know that it is fairly straightforward to modify the plate that I have to fit another chuck, but that would still leave the mystery.

Dimensions:
OD: 4.25"
Bolt pattern: 3.625" bolt circle, 5/16-24 threaded hole, qty 3 evenly spaced
raised centering boss: 3.125" dia, .11" height

I assume that it should have a 4" chuck on it.

At some point in it's life, someone used a wrench to remove it from the spindle. Other than some scarring from that (which will clean up with a bit of stone work) it is in excellent condition.
The image with colored areas is one I used asking some questions on another site but does show my dividing head. in it you can see the original back plate behind the chuck. There was a very poorly made adapter in between that introduced a lot of runout.

FWIW, my 7 1/2 has a 3-jaw scroll 4.25" dia. Skinner chuck on it and it does not match your backing plate.