I've not seen this before either - it's unusual - will check in the records at Kew, probably next week now and should be able to get some pix. Registrations are, mostly, for the shape rather than decoration, but it's not a completely hard and fast rule. I'd suggest that if the No. was etched then its appearance would not show as mat like this - acid leaves a clear surface on the glass, usually, which was why its use as a polishing agent - on cut glass - became commonplace after c. 1920 ish. This No. is probably wheel engraved.