They're actually different. The 53,51 handguards are longer and wrap around the trunnion slightly more but the mount dimensions are the same. I don't see why it it wouldn't work.I can't say for certain but I don't see why not. They're the same size hand guards.
I wonder if an HK53 hanguard would fit on an MP5 though. They sure feel better in the hand IMO. I had an MP5 handguard on my 53 and switched it out, and man it fills the the hand out so much better.Interesting. Learn something new everyday. Try look do similar it isn't funny.
That's what I've found on the UTG. Also, the push pin holes don't exactly line up properly. With a few mods it looks like it could be made to fit and it's cheap enough that you don't feel too bad to hack on.I have a 53 wide and a mp5 tried it. its too long in the back of the forearm from the part that catches the trunnion to the end of it by the magwell.