The SL8-6 for U.S. import has a narrowed magwell that will not accept hi-cap (G36) mags. Other countries, such as Canada, get other variations that do accept hi-cap mags.
All SL8's have the thumbhole stocks, has nothing to do with destination coutry.
The main legal difference between the SL8 and the G36 is the SL8's inability to accept a full-auto G36 carrier. There are other differences, of course, but the receiver's inability to accept a F/A carrier is THEE single difference that can not be legally modified on the SL8, all the other differences can be legally changed in one form or another.