Numrich arms? Looks like they're out of stock on the yoke but they do have the block! Maybe a Western Field or Wards version would use a similar yoke?

https://www.gunpartscorp.com/gun-manufacturer/savstevspgfld/shotguns-sav/59a

Maybe even a pump shotgun would use a suitable clamp like that?

Here's one from a J.C. Higgins M10 B/A shotgun that might do until you (ever) found the right one. Just a thought.

https://www.gunpartscorp.com/products/1405050A