::shrug::
I'm not the huge collector type, and the 6520 isn't even something like the SP1 that it might be too sacrilegious to modify even a little bit... once again, the 6520 has been available in various forms for quite a while, and there are a lot of them out there.
Based on the lack of a drain hole in the lower receiver "fence," I'm betting it's not drilled for a pivot pin detent, though it could be, if it were mine, I'd spend the extra cash and send the lower out to SAW or ADCO, have them mill the sear block flat, and if necessary, drill a pivot pin spring/detent channel in there, and use a full weight carrier and "H" buffer.
::shrug::
It's a minor issue at best, and chances are, you'll never notice any difference whatsoever if you don't do so, but a) I just don't like the cut carriers, they chew can up the buffer face, and depending on the carrier, can hammer the firing pin retaining pin, and b) the 6520 was something of a parallel development to the RO727/M16A2 Carbine family, which, through a very circuitous means that I won't go into in depth here, eventually culminated in the M4/M4A1 Carbine as the most mature variation thereof, and while the M16A2 Carbines were very close, the M4 introduced some refinements that would later get integrated into commercial/export carbines.
Given that it's a pre-ban, too, I'd probably swap out the extractor spring and buffer, because chances are it's the older version, before the ECP to the M4.
Minor changes, but ones that would essentially do little more than to update the functional components to the "last gen" 6520 specs anyways.
That being said, I probably wouldn't touch the factory staked two position receiver extension/lock ring assembly, which might negate the lower work depending on the fixtures, but that's just my own picayune tastes.
~Augee