Battery connection on new-to-me 1974 MKII Commando

Thomasa said:

It seems unlikely to have been converted to negative ground. If one were to convert to negative ground would you not need to reverse the zener polarity?

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You would, and the previously mentioned rectifier.

Thomasa said:

The stud would have to be electrically isolated from the frame/z-plate and a wire run from the stud to the rest of the electrical system. But then you'd lose the heat sink. That would be a dead giveaway... unless there is a negative ground Lucas zener that I don't know about, which is entirely possible.

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Yes, the 1979-on negative earth/ground non-electric start* Triumph twins had a low-output 3-phase RM24 and negative earth/ground (Lucas 49589) Zener.

*(The electric start models had the high-output 3-phase RM24 and negative ground 'AC' triple-Zener assembly.)

Are the Triumph zeners still available? I've heard that the positive ground ones are not.

Thomasa said:

Are the Triumph zeners still available? I've heard that the positive ground ones are not.

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There's the negative type in the link I posted.
Positive (Lucas 49345) are perhaps not so easy to find.

Thomasa said:

When you do finally put the battery in, invest in the factory hold-down and the associated rubbers. They are available and not expensive.

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Velcro works great and is a decent isolation for vibration. Apply velcro on the bottom and rear.