Many thanks gents. Much appreciated. I've had quite a lot of HBM Co scopes go through my hands over the years, including some earlyish examples, but now wish I'd noted all of their details before selling them! I'm pretty sure they'd stopped the practice before 3100, but interesting to know that at least 57 would seem to have been dated. It's only a minor detail in the No4 T saga, but something I've been curious about.
Cheers.