George has it right. I cross referenced it against the 23C Manual since the lens board area is identical.
The manual states the following:
  • Screw Tapp - #4BTx3/8" Long Pan Phillips
  • Part/Cat. No. 538-13-12-31 (only useful if ordering from Beseler)

If the hardware store does not have it, then try Beseler. Or maybe someone is scrapping one and can scavenger the part. Or, if you have a friend that is handy with metal working, they could tap it for a regular machine thread.