There are 2 distinct rear foot peg brackets used on Sandcasts. Early bikes had the inner and rear reinforcing plates as well as the two inner round cotter pin reinforcement plates hand welded in place on the outer bracket, whereas later brackets had all these plates spot welded to the outer foot peg shell. I have no information on when the later spot welded type was introduced. I'm led to believe only early bikes had the hand welded type. Maybe owners could check their rear foot peg/rest and advise the VIN so we can get some idea.
I hope the photographs are clear enough. Sorry about the quality of the early foot peg but they are fairly hard to come by