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Bear with me for some extensive notes! I hope this can be helpful to folks trying to ID these.

There seems to often be difficulty in separating Osmia taurus and Osmia cornifrons males; I believe this is O. taurus, and so I wanted to try and determine which characters were most reliable in distinguishing these species. This is the first time I have encountered a male O. taurus so I appreciate any feedback or discussion of their ID, and confirmation that this ID is correct. I've posted an O. cornifrons male as a direct comparison, with photos from the same angles, here:
https://www.inaturalist.org/observations/157939181

This specimen stood out next to the series of O. cornifrons males as being larger (11mm next to ~9mm), more uniformly and densely hairy with on average longer hairs, and a more tawny/buffy color (though not orange or foxy as descriptions stated). The last photo in this series shows a lineup of males, with the O. taurus on the far left. However this likely falls within the range of variability for O. cornifrons.

DiscoverLife lists two main features to separate these species. Firstly, hair color, with O. cornifrons having generally paler hair with intermixed black hairs on the scutum, while O. taurus should have more rufous hair with no black hairs on the scutum. While as mentioned above, this O. taurus was more tawny, it did actually have a handful of black hairs on the scutum, restricted to the areas near the tegulae, or at least between the tegulae and parapsidal lines. There were no black hairs on the tergites. Black hairs were intermixed on the face for both, with O. cornifrons generally having a few more in the clypeal region.

Secondly, there is a description on DiscoverLife of the unpitted band on the clypeus:
O. cornifrons - "Head, clypeus apical rim, the central straight portion, shiny area forming a narrow, unpitted uniform band across the front of the clypeus, usually extending to the far sides, except in a few individuals which may have scattered pits on the far sides"
O. taurus - "Head, clypeus apical rim, the central straight portion, very shiny and unpitted in the center, but at the far edges of the rim before it curves back towards the head, this rim is invaded by the same pattern of heavy pits and roughness found throughout the rest of the clypeus this roughness usually extends near but not quite to the rim, thus the shiny unpitted portion of the rim forms a SEMI-CIRCLE rather than a uniform band as in O. cornifrons"

I found this description somewhat confusing, and could not identify these features on the specimens. All the specimens in my series appeared to have an unpitted band that stretched across the rim of the clypeus, with a few pits encroaching at the point where it curved back towards the face, then becoming shiny again past this point, essentially creating a semicircle. It was difficult to get photos of this, but I included two from different angles for each specimen. The O. cornifrons had a wider band especially in the center, with some specimens having this central part depressed (can be seen in the specimen photo in the other post). O. taurus had a few more pits encroaching on that curve - maybe 6-7 vs 3-4 in cornifrons. O. taurus had a minute notch in the center of the clypeal rim, whereas only 2 of 10 cornifrons specimens had a vague notch, the rest having a sinuous or complete margin.

However (and TLDR, skip to this part!), the most compelling and consistent difference was the shape of the gonocoxites. Luckily, these were extended on almost all the specimens. On O. cornifrons, they are bulbous, with the apical projection facing almost horizontal outwards - I liken the shape to a closed fist with the thumb extended straight out (see the other post). The projection barely rises vertically above the horizontal plane. In this specimen which I presume to be O. taurus, the gonocoxites are not nearly as expanded, and do not have a flat plane, instead being sloped diagonally downwards, and the projection faces diagonal upwards and out, like a closed hand viewed from the side with the index finger pointing out, i.e., the projection rises much more vertically away from the subapical area. I've showed photos from two angles, the second being a pretty poor photo (legs were in the way) but I wanted to show it from as close to the same angle as possible as the O. cornifrons for comparison.
In the other O. cornifrons specimens I looked at, there was absolutely no variation in the shape of the gonocoxites (as would be expected).

The difference here was so striking that I have to think this would be the most reliable character to separate males of these species. This is consistent with what is stated and illustrated in Yatumatsu and Hirashima 1950. Genitalia illustrations and good specimen photos of both species can be found here:
https://idtools.org/exotic_bee/index.cfm?packageID=1185&entityID=9084

Please let me know what you think!