I'll be the first to admit that I was perhaps somewhat narrow in the search. I began checking casting dates. In particular the late 1920's and early 1930's, and went off the AC and AD lists only. I probably should have looked at other letter codes to verify. Then, for fun (?) I checked online auctions for photo and descriptions of 15's, and tried to keep on this side of the pond. So the findings may be flawed. The production dates on the 101 probably hampered this since it was a US only machine and the 201 supposedly replaced it. If the 201 is being made in 1932, then it took Singer several years to 'replace' the 101 as both appear to have been being built concurrently for several years. As helpful as casting dates are... they don't differentiate a run of 15's from a run of 15's, leaving all sorts of room for speculation.