[Uriah]

Guarantee you're good to go with that rifle and serial number range.

The parts will be stamped. You're likely to mostly find FMP, which were manufactured with the HK tooling/equipment. Some HK marked parts are possible, but from what I've seen, that's generally just the backplate, as aforementioned, and the locking piece.

The heavy barrel is correct, and was originally target crowned. I'd just be sure the threading job was done well.

Verify the bolt gap measurement if able.

 

[Uriah]

It takes some getting used to, but is a breeze after you've been doing it a while.

The backplate is a very precise fit. I generally just make sure the entire lower is pushed forward and up as far as it can possibly be, and start by putting the bottom edge of the backplate just over that part of the lower that extends back, then sort of lifting up/tilting forward...side to side twist as well if need be. Just look at all sides as you try pushing it on, as everything pretty much needs to be exact.

It's amusing how seemingly difficult reassembly can be until you get the hang of it, or something finally goes into place. It's like, F***K, now why was that so difficult?

I've certainly had my fair share of trials early on with the locking piece/bolt assembly, as well as getting the stock back on.

Just keep working at it, and don't force anything.

 

[Uriah]

You can see these guys hassle with trying to get it back on, heh. Not the best vids by any means - just came up with a quick google search while trying to find something visual for you.

Stripping/reassembly of a G3 style rifle - YouTube

G3 disassembly/reassembly - YouTube