Unknown British Primer Clip

I am trying to identify this British primer protector clip.
The corrosion makes the markings a bit unclear but seems to be:
NWNco 1940 No55 (or 53?) Mk1. Made of steel.
The rim has to be 39.5-40mm wide. It fits a Russian 30x165.
I cannot find any British shell with a rim this size that needs a protector. Any ideas?

Clip Date C Clip No D