Apologies everyone, it was certainly not my intention to open a can of worms or indeed waste anyone's time.
@ Wiltshire Moonraker - the photo of the alternator was the only one I had to hand and although not the current alternator in the boat (which had been replaced like-for-like – the new one is black but the same casing and connections), in my ignorance I thought it would be relatively straight forward to identify it. Perhaps in hindsight, I should have waited and posted actual photos. However, I have already learnt a lot from my original post. Which is the point of the forum isn’t it?
@ churchward - it is indeed a BV1505 in glorious red paint, the majority of which is still on it. It always makes me smile when I call Beta for spares and they ask for the serial number. They always reply "Oh, that is an early one". I understand it to be a 70amp alternator and I do have a sterling battery device that might explain the other wire?
Perhaps it would be useful if I took a few more details photos to close this thread out.
I would like to thank everyone for their comments so far and apologies again if I have wasted anyone’s time.