I think they are!When I was going through Papkes stuff at his estate sale.I saw bumper brackets that were labled 49&50.I didn't see any 51's.Like John said.Matt would probably know!
I know from experience that the aprons behind the bumpers are a lot different between 49/50 and 51 as I had to do a lot of trimming when I put a 49 or 50 nose on my 51 and used the 51 bumper and brackets. That would indicate that there is quite a difference in brackets.