[TR] What is this part?

[Randall]

It's the front cover for a TR2-6 gearbox, the part the throwout bearing
carrier rides on.  Photo is kind of confusing, as it has been coated in
Cosmolene or similar, which has bridged the opening where the input shaft
would normally protrude.

Ad claims it's the later, shorter version, for use with the TR4A-TR6
'diaphragm' clutch.  Hard to tell from the photo though, it kind of looks
like the earlier, longer unit to me.  But if so, it can be easily modified
with a hacksaw.

-- Randall