The only known K-6s I see were being tested in late 44, but there are no work numbers listed for any 'production' of K-6s.
I'd rather see the R4 (gondolas) option added to the K4. It didn't see 'widespread use' but it was available. Further, there were production models delivered with the 20mm MG151 hub cannon in place of the MK108 - not all that many mind you, but enough for that option to be added IMHO.