Ammunition held at Hockering M.U. 231

Hello,

I am currently studying Maintenance Unit 231 based at Hockering and whilst looking at a plan of the site I came across some names for ammunition that I don’t recognise.

The first is 3″ S.B. Ammunition. Was that for the Smith Gun?
The second is 20 M.M. “X” or “Z & Y”

Also does anyone know what was kept in component stores?

Any help anyone can give will be gratefully received!