David, my dementia has kicked in again! I even captured Lee's photos for my records...how could I forget??? He even had photos w/grips off. Of all the things I've lost, I miss my mind the most. Thanks for jogging my memory!
The only rebated frame post wars I have are 32s. So the serial # won't help you but just for reference the earliest unimproved I frame I have with rebated frame and leaf spring is: 545654.
The 38 RPs I have are prewar and under your 'high #' of 54474.
I know I've seen many post war unimproved rebated 38 S&W RPs as well over the years. So with your enlightenment about rebated 1st improvement I Frames, I'm convinced there was probably no gap in rebated I frame RPs until discontinued with the introduction in 1953 of the 2nd improvement I frames.
So what I'll be focused on observing now is what I refer to as the 'last transition post war model' with rebated frame, to back up my theory.
These are defined by being 1st Imp., but having ramp front sight w/barrel rib, grooved trigger and 1st use of the 2nd style flat latch.
Lee's photos in his thread here:
Transition 32 & 38 Regulation Police
show what I call 'mid-transition post war models': they are 1st Imp., still have 1/2 penny front sight, have the 2nd style post war semi-sculpted thumblatch (which preceeded the flat latch), and they introduced the grooved trigger on non-target guns.