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
All G4s came with the plain grey cap with option for a remote primer set up. I had a Mayfair low arch with the grey cap and remote primer button bolted to the deck connected by a tube to a second outlet on the grey float cap.
Victa had to engineer an opening into the right of the Mustang engine cover as a result of the new LM carby in 1978 with it's white built in primer cap, so it would face outward.
The left hand snorkel wasn't a problem as the Mustangs all came with an outward facing remote primer.