Honda's literature gets a little blurry sometimes, as far as parts vs. serial numbers...My K2 has a 90% K1 engine, but an early K2 serial number. Inside, it's K2-later clutch, K1 final drive, K1 rings (1-piece oilers, saw these clear into the 1972 K2 season on others, too), K1 cam & "holey" sprocket, K2 18T countersprocket, K2 carbs with 110 mains (K1 was 115 mains, K2 later was 105 mains in the 657B carbs). Spark advancer was K2, with shortened advance and lighter springs.
The 3-piece rings allowed looser bore straightness. They would still seal well that way, but not at real high RPM (>10,000). We would seek out the 1-piece ring and K1 pistons for that work, otherwise the rings fluttered a bit 'way up there, and burned oil as a result. The bores must be dead straight to have trouble-free 1-piece oil rings, though.
The oil pump: there was a change in the K1. I seem to remember it was the dowels and their related O-rings, but I don't have notes as to what, exactly, was different.
