This is what the bulb holder looks like, the larger 4" lens just screws into the threaded metal ring which gets fixed to the vehicle. This is a stop/tail light unit, has a two pin bulb holder. The indicator will have a different bulb holder inside, single pin.
The front lamps could be a similar type, ie an indicator unit, single pin bulb holder, but with a 5w bulb and clear lens. Or the early trumpet type.