Hi Daren,
Very difficult to answer your questions, because the 123-5 doesn't appear in the usual B&B literature, which would have us believe that the 123 carburettor wasn't fitted with a pilot jet, and yet the 123-5 clearly is. Perhaps it was made specially for Douglas?
Also 1928-1929 crosses the boundary between the old firms (AMAC,B&B, Binks) and the new (AMAL). Technically a B&B wasn't available in 1929, yet I bet they were supplied. Were some sold with AMAL float bowl lids? I don't know, but yours it the early AMAL type. I'd guess they had B&B lids originally, even if supplied in 1929.
Parts? I've suggested asking seller b561 on ebay before: he makes lovely parts and reconditions carbs, though more AMAC than B&B. Here's an example:
https://www.ebay.co.uk/itm/143927411703Here are the relevant parts lists, even though the 123-5 is not mentioned specifically.
Cheers
Leon