Hi wce,
those caps were fitted to the high arch with the cowl (Mustang style) and they didn't want to cut a hole so the snorkel could come up on the right hand side. The later covers had a section cut out for the snorkel. By putting the snorkel up the left hand side meant they had to turn the carby the opposite way around and this put the primer bulb out of reach so they put a primer bulb on a little sheet metal bracket screwed to the base just in front of the carby and ran the hose from the back of the primer around to the primer cap
