[TR] Triumph Radio question

[banc8004 at comcast.net]

I have read from this list that radios from earlier Triumph cars (and others of that time) have a ground selector to set the radio for operation in pos. or neg. ground cars.

I have an old Triumph radio (two knobs, 5 push buttons). It is branded Triumph and has the 'book' logo in the display . The display has a 5 to 16 scale, and is AM, I assume. It has is no ground selector.

Written on the case:
4TBTR 009342

Is it safe to assume, as there is no alternative ground setting option, that this radio is for a positive-ground car?

It has three wires. One black, with a bullet connector; two speaker wires Brown and Brown/Yellow.

I want to test the radio to see if it works, and if so, find it a good home. It is in surprisingly good condition, compared to the junk I've seen on ebay.

Can anyone confirm this is positive ground?

Cheers,

Brian