Ah, the 1889 test rifle. That makes sense, it would have been a hand made rifle with no tooling, thus a lathe turned and bored receiver, but as only one or two were made, and they were not, in fact 1891s, it won't affect the rule: Hex receivers only. But it does make a point: the round receivers were easier to machine, not the hex.
The 1889 was also probably a single shot! Check out C&Rsenals new video on the M-N.