It's one of the first piston AR. HK416 is not the first piston driven AR as H&K would want you to believe.
My brother-in-law was in service served as a conscripted soldier, they were still using T65K2 (At the time, professional soldiers had switch to T91 already, T65 is still kept for reserved) AFAIK they've move to T91 completely.
From what he had said, T65 was a pain to clean (the piston system had a lot of small parts unsuitable for fieldstrip in the dark)
Apparently the biggest change with T91 is reduction in number of parts for the piston.
https://www.youtube.com/watch?v=k9UGmGU-h_c
In Mandarin: T65K2 ('M16'), T86, T91