Looks like an FN7 taboo track to me,if you could do some working out of what that internal curve diameter is, we could say for sure.

Hi Martin,

Having looked at many pics now I can conclude that it’s a Mk III Stirling taboo track for the FN50 dorsal turret. While cleaning it today strangely roundel blue paint came to light. That puzzles me.

Cheers

Cees