Thanks for the replies. Yes I originally thought it was a Smiths too, and obviously it was fitted to to the original 260/289 Smiths-instumented cars.
I found the Lucas parts listings with the model interpretations. Roger, it's definitely a BM4 that I'm after (and yep, BM means base moulded, and "4" means it has a damped movement).
AJ, I now have a list of eleven Lucas 'BM4-family' ammeter model numbers that are correct or could easily be modified to be correct.
They do appear to be rare though, but where's the fun if there's no chase?
Cheers,
Glen