Story,

Perhaps they were hammer forged that way? CZ does this with current rifles. The mandrel is simply tapered to allow the swaged bore to be created. Perhaps the same method was used in these older rifles?

A swaged bore allows for a tight fit on possibly wide ranging tolerances in ammunition, without raising pressures to a dangerous level. Honestly, I've never heard of it being used on something this old, but then again, I haven't heard of a lot of things!

(It had to start somewhere, and I seriously doubt it was the current CZ's idea.)

Perhaps someone might know more?