To date I have classified this stamp as a CL2B, but upon closer inspection, I find that this is a different clay paper from 1955. I am calling this stamp the 25cCL3-SO from here onwards. This stamp was not issued as a regular printing. If you come across one without the overprint it is a proof or a very rare missing overprint example.
This strip of the 25cCL3-SO has variations of the overprint and an unverified plate variety.