I wish I had definitive answers for you? But I don't. What I "know" is that there were several interim variations for model T side and tail lamps for the early 1915 model/style Ts. A few are known as used on the very early center-door sedans and folding top coupelets. Even there, they began with spade mounts and then moved to bolt mounts. What really confuses the issue, is that there were many after-market newer style replacement oil lamps in the era that were made for several years, but somewhat resemble the new Ford lamps of 1915. Knowing which were actually used by Ford and which were not, is a problem for which no reliable references exist as far as I know.
The very early (pre-open car) 1915 Ford lamps were distinctive looking with their smaller lenses and fatter bezels. However the earlier closed style cars were produced in small numbers, and therefore the oil lamps were also made in small numbers. As production issues for the new style 1915 cars were being resolved, and production of the new style open cars slowly began, the oil lamps were also evolving.
I did some research a few years ago, with my limited resources, and checked with a few people that knew more than I did, and wound up with some theories (?) and speculations, but few solid facts.
I happen to have the tail lamp that appears to match those sidelamps. I am planning to use it on my early 1915 runabout. It had been altered to be a combination oil tail lamp and electric brake lamp. It was nicely done, and I made it even nicer with some restoration and new cloth covered wire.
What my research did find, was that this was likely a lamp used by Ford around December of 1914 and maybe a few months more. I have seen several others in recent years, but they were likely not used on any large number of model Ts. Royce P did quite a bit of research on the early 1915s. If I recall the numbers correctly, Only about fifty 1915 style open body model Ts were produced in December of 1914, basically all runabouts (maybe one or two touring cars?). In January of 1915 less than two thousand 1915 style open cars were produced (compare that with the serial numbers for the month and realize all the others were 1914 style!), mostly runabouts (a couple hundred touring cars). All other production of open cars was in the 1914 style in those months. Production numbers began climbing through the next two months while more and more open cars were in the 1915 style and less and less production in the 1914 style. The last of the 1914 style open cars were believed to be produced in April of 1915 (although some believe a few even in May?).
This interim style lamp, or something very close to it, was likely used as late as February of 1915 on 1915 style cars which were still the minority of factory production.
While similar in appearance to the later common 1915 style sidelamp? The only thing that is nearly the same and will interchange with the common later 1915 lamp is the lens and bezel itself. The body, inner and outer chimney parts look similar, but are slightly different sizes. The entire inside is very different as is the bowl and its locking method. Even the brass cap on top is different! It is slightly larger in size, and uses a size larger screw to attach to the chimney.
Era photos rarely show the lamps in well enough detail to be certain. However, this style does seem to show up in era photos often enough.
But I think these look like them?