Another uknown item

Yet again i have come across another mystery item found at binbrook.
Having probs with my camera at moment so cant upload a picture.
The item is plastic and circular in shape and i think its some kind of protective cover as its blank 1 edge.
I would say its about a inch in depth buy 3 inches across its entire diameteer and is black and made by thin plastic.
Markings are as follows

Remove cap before loading
Section 12S
REF No 1141
EL.
57 in a circle

Would this be a blank for a canberra starter cartridge ? and if so has anyone got any pictures of a cartridge with the caps fitted.

Hope someone might be able to confirm this for me

Cheers