yes most definitely H with 58C in the centre then a W on the right.
Actually I do have some cream porcelain insulators, was told they are BT ones
HW, well I'm stumped on that one-- so it's a new maker apparently. Thought I knew all the big ones. Maybe they were little fish? Who knows? It's not that unusual for me to find glass made buy yet another vault light company. Vault lights were huge for a time and lots of people made them....
In the UK, glass insulators were tried very early, but they quickly switched to porcelain and it's been all "mud" ever since.
You're presumably more interested in glass? I have lots of glass insulators for trade. If you haven't seen what they look like, check out this
gallery of colored glass insulators for starters...
--ian