"Extra Part" , Needs identification !

[jl46]

Hello All,
After rebuilding our 1966 mk1 MCS brake servo we found an extra part.
We believe it is a part from the servo.
It is 1/2 inch in diameter with a square metal attachment. The metal has a small inscription that looks like an "A". In fact it almost look the early Austin logo or script.......
Attached are some photos. Your help is greatly appreciated.

Jim

[nick rogers]

It is not listed in the diagram as a separate part but it fits inside the filter of the air valve on a Cooper S servo.

[jl46]

Thanks Nick.
Very much appreciate your help.

Jim