If you search the net with the string “james dixon sons sheffield catalogue” you will find that there are quite a few editions out there, but availability may be very limited. I do not know the name of your HSMc manufacturer but there is no central registration system for electroplaters.
A1 means best quality but there is no objective standard for this and it simply means that the manufacturer judges it to be their own best quality.
Thankyou so much for your reply. What a fascinating world silver collecting is! I think I may be getting the silver bug.
Electroplating seems a world of its own. I will endeavour to find a catalogue for James Dixon. So interesting!