Actually, in terms of fork legs, it'd be more accurate to state that the K0/HK0 was the only model that did not have hydraulic fork legs. Consider these "pogo stick" forks as a complete assembly and a world unto themselves, with the handlebars being the only parts that directly interchange with any other models.
K1/HK1 is the only "gatorless" type, with metal lower covers, rather than ruber boots, a.k.a. gators.
In terms of parts interchangeability, K1 HL ears only fit K1 chrome rings, to which the lower covers are also matched. K2-`79 HL ears & chrome rings have the upper attachment points for the rubber gators...they also mount beneath the lower tree. Turn signal mounts were added with the K3 model. K1 lower fork legs are the only version that lack a shallow groove for the bottom of the rubber boot/gator. Otherwise, K1-`79 fork legs are the same and all parts interchange between them. `79 top trees are different from K1-`78, having different spacing for the conventional bar risers...`79-`94 are the same. 1980-later fork legs are conventional sweeper-style and did not come fitted with tubular HL ears (which are also upper fork leg covers) or the K1-`79s.