Originally posted by ChrisPlatt A few things I can add which don't appear to be on that linked site:
- for a time I had a used "made in Japan" Carl Zeiss Jena-labelled B series 75-300 zoom, which is not listed - at the time I thought it could have been made by Sigma, but I now see that maybe it is more likely to have been made by Samyang! Anyway, was not impressed and soon sold it on.
- I still have Sigma (not Samyang!)-branded 28-70 and 70-210 B-mount zooms - they came together in a Sigma-branded kit case (now long since gone).
I just compared that Sigma 28-70 with a similar but much later 28-70 PKA-mount version, and there is a lot of resemblance - notably the markings around the front element - and so I think that the B-mount version is almost certainly a genuine Sigma product.
I also still have a BX-10 and a BX20S, both with motor drives, and the latter was my favourite B-mount body. OTOH, I briefly also had a used BCA which I bought cheap and then traded in for something else as the above bodies were more useful.