The changes definitely came through in 1905, so until we know better let's use the 1902-1904 dating... For that matter, the guard seems to be consistent with the oscillating fan as well in that period...
The R & M image I zoomed in the 2nd Post is the same pic you used in the 8th Post. When Curt didn't like the answer, I explored deeper into the subject and came up with the possible 1904 oscillator; my whole point in posting at all was to register the guard as, seemingly, Robbins & Myers, regardless of model or year, and that was the goal in the first place.
So we've narrowed the possibilities, and I've learned some stuff of which I'd otherwise be ignorant... nice.
It would be great to see more photos of these super-early R & Ms.....