Stromberg Gasket

What is the little cut-out on this gasket for? The ones i have dont have it

That is aligned with the channel drilled in the carburettor to feed the bypass valve for Stombergs fitted with it . Most certainly you don’t have these ?.

As far as i know, all the Jag V12 strombergs have bypass valves. Interesting that the non-bypass gaskets are supplied.

But you say the one you have doesn’t have it. In that case the bypass input was blocked, either on purpose by the PO (to avoid problems with defect valves) or by mistake.