Based on SN and per Thiele, Fabrikationsbuch/CZJ..., this is part of a 1000 lens run in 1948 that was part of a larger set of M42 runs, but some of the specific run of 1000 your lens is listed under included some Exakta mounts. So it was likely either M42 of Exakta mount, or maybe some intermediate result that got out of the factory. Best of luck.
I have the lens in Exakta mount, and it is very nice. It would be great to have one in M42!