E. Robinson Lock Plate Question

[RSMorris]

I just got in the mail this 1863 dated E. Robinson lock plate. It has a 145 stamped on
the reverse side. Anyone ever seen this before?

I've looked in the Moller book but it says nothing about these kind of stamps

 

[Papabyrd]

Probably an assembly number

 

[RSMorris]

Probably an assembly number

I have seen several E. Robinsons and those do not have a number stamped. I know several isn't very many. I do have lock plates from other contractors and none of those have numbers stamped. Have scoured the internet and the many books I have and none of those show a single lock plate stamped with a number on the reverse side of the plate. While I was at the Franklin show, Ed Knisely, the parts guy, he calls me the Needham guy, had probably 50 to 60 lock plates there and none of those had a number. That's why I am curious.