The M50, is roughly speaking the graphite/epoxy resin cased equivalent of the old reliable metal cased 109 "Adjustable" second stage and both use the same poppet as does the non adjustable and unbalanced G200.
The adjustable feature of the M50 gives it an edge over the non adjustable G200 in terms of flexibility. The M50 and G200 differ in regards to how the air barrel is constructed and the G200 is much more similar in that regard to the G250. Consequently the G200 can be upgraded to use a balanced poppet, in which case it becomes a G200B, where the M50 cannot be upgraded with a balanced poppet.
The M50 is very similar to the Polar, which is optimized for cold water and is also adjustable. The M50 came in both primary and octo versions with the difference being the color of the purge cover and how they were tuned. The M50, like the original metal cased 109 Adjustable late in its career, was an intermediate level second stage that combined good performance and adjustability with a simple and very reliable unbalanced poppet design.
All of the above mentioned second stages were good designs, but SP moved away from the unbalanced adjustable downstream design for what I suspect were marketing reasons. They changed their marketing strategy for middle level regulators and dropped the unbalanced adjustable second stage option in favor of a smaller version of the unbalanced non adjustable R190 second stage.
I am not sure on the start and end dates but the M50 was produced more or less in the 90's and, compared to other SP second stages, had a relatively short production run. It would have been paired with regs like the Mk 10, Mk 10 Plus, Mk 15 and Mk 20 during its production run. As such it would have represented an intermediate priced regulator with a better than basic first stage paired with upper end first stages like the Mk 15 or Mk 20 or with the on it's way out but still excellent performing Mk 10/Mk 10 Plus design.
They are, relative to other SP second stages, pretty rare and I only see a few per year for service.