Amazing community assist - thanks gang! Husk came through with not only the plate I needed, but it's matching pointer, screw and spring. Steve H. in N.Y. then came through with the bead dimensions so I could machine and oil blacken a bead to repair the damaged pointer Husk sent me.
This will have a place of pride on the 1913 SMLE MkIII I'm fixing up. It went through a 1920's AustralianFTR and all the MkIII bits were removed at that time - I'm putting them back because the furniture is set up for them and I much prefer the WW1 as-issued trim.
Pics to follow when the project is done. I attached a pic of the repaired volley sight in the rifle's forestock.