Somewhere recently (it may have been in Keppler's "The Pentax Way") I read that with the Spotmatic etc. viewfinder optics the 55mm lens gave a true life size view of the subject (not sure why that matters) whereas the 50mm only gave 0.9X life size. The way I read it was that buyers were invited to choose between life size or half a stop faster.
I have several of both focal lengths but rarely use the 55mm as I find that for the kind of things I'm photographing the difference in viewing angle, albeit small, is often significant. (In case anyone asks why I'd have lenses I don't use, I'm also a collector and they turn up attached to bodies!)
Slightly off-topic, I was always amused by the adverts for the then "new" SMC lenses which showed two supposedly identical shots into the sun, one without an SMC lens and one with. The reduction in flare was staggering - until you looked closely and realised that the scantily-clad yound lady had moved over for the SMC shot, obscuring more of the sun than in the other shot!