Strange timing, tonight I just dremmeled off the muzzle nut on my 1990 era PARS XG3S (Potuguese). The HK 91 is just the HKs model name for the semi automatic version of the G3.
All HK 91s are pre 1989 import ban. The '89 is a very strict ban, and allowed no 'evil features' whereas the '94 allowed one. They imported some postbans ('89 ban but pre '94). The postban semi G3 was named something other than 'HK 91'. Those (and clones) had thumbhole stocks and either a muzzle nut or a naked barrel. I can think of only two HK licensed clone builders, the Greeks and Portuguese. The import of these stopped fairly quickly. I assume sales were too low to justify further import while domestic stuff was still cheap and free of restrictions.
The G3 has 17 of the 20 922r parts if it has a threaded muzzle. You must replace 7 parts to achieve domestic status and be free of the '89 which did not sunset. Decent domestic furnature is totally missing, so one has no choice but to add US parts to the magazine to keep decent furnature.