Some pointers/advice on what to look for when inspecting a used HK91.
Make sure it's German made. Check bolt gap. Check for bubba things like grinding the bolt head. I have see that on a real HK91. Check the seating of the magazines. Try different magazines. if possible take it apart and make sure it comes apart and back together fairly easily. Check the rollers and make sure they move freely.
Everything will be date coded This 91 is an IB date code so it’s an 81 just remember A is equal to 0 B is equal to 1 and so on a lot of guns they skipped J but you can find J on lots of accessories This is the side of the magwell.