Is it a raider 10 or just a raider 1 or 2 shot? As far as I know, neither of these had a pressure gauge, only the Raider 10.
Could be one or two reasons why yours is a .177 when it says .22 on it. Someone might have decided to change calibre and had it professionally converted (I think there was a time up to when Webley went bust that the .177 was in very short supply, so the above might have been an easier option for someone).
Or the shop itself when selling from new might have converted it to .177 for a customer as they couldn't get hold of one new.