I wonder if the Maltby FTR marking is used in a slightly different context, perhaps rectification of early receivers that needed work to bring them up to spec?

Maltby did have some totally understandable quality issues as they got production underway.

The 1942 date on Simons, could have been an early receiver that failed inspection, put aside and rectified (stamped FMR), to rejoin the production line as it massively ramped up in 1942.

I would love one marked that way, sucker for a Maltby....