This is a neat and lovely camera, but the price has always restricted me from buying one. There was the old SWC with chrome lens and no T*, which was made until around 1973. Then there was the old SWC with black lens and T* (essentially the same camera apart from the coating and the colour of the lens), which was made during the 70s. In 1980 the SWC/M appeared. It had a lowered tripod socket to allow mounting Polaroid magazines, apart from that, not very different from the old SWC. In 1982 the SWC/M was changed to the CF-lens barrel.
Somewhen in the mid 80s the SWC/M got a newer finder and in 1985 the spirit level on the body was dropped and integrated into the finder instead. In 1988 the SWC/M was succeeded by the 903SWC, which was built until 2001.
The Biogon lens design never changed during that period. It was not redesigned before 2001 when the 905SWC was launched.
Essentially, there is not much difference between the old SWCs and SWC/M with old lens barrel and the SWC/M with CF-barrel and 903SWC.
There is sometimes an issued reported with SWC/M cameras´ shutter leaving slight scratches on one inner lens element, this was solved in the mid 80s with a shutter modification. It was done somewhen between the introduction of the new style finder and the loss of the spirit level finder on the body, so cameras that do not have the spirit level on the body should not suffer from this issue. It was only a minor problem before though. These scratches can occur with cameras before the modification was done, but they do not necessarily must. I doubt that they will effect image quality at all.