Straight from the G36 Armorer's Manual, about the only change they mention is that the pin that was used in early G36 cocking handles was replaced with a non-removable rivit. The pin allowed for disassembly of the cocking handle, spring, and centering device, but I think HK revised the design since you don't really need to take those parts out to clean the rifle thoroughly.
Apart from that, there are no other revisions that I've seen or read about to the G36 that's available today.
Along the line of the forearm redesign, I actually have 3 'series' of G36 forearms which have evolved over time. The first was an early edition with more squared corners all around, the second had more rounded corners, and the 3rd has rounded corners and the 'hardpoint' inserts at 3', 6', and 9' o-clock positions.
There are many different handguard which could be swapped with a single pin, including the AG36 mount with double rows of cooling vents.
Stock variations obviously include the shorter 'C' stock, and the cheek rest stocks, as well.
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