"Stock" headlights were simply the Beetle buckets installed from the back of the light pod. Holes were drilled so that the adjusters would poke through the fiberglass. The light bucket itself sticks a little bit through the fiberglass so that the headlight keeper ring will hold the bulb in place.
View attachment 5934
As for the market lights, my lenses had been cut flat, then attached with a pair of screws through the reflector and into the fiberglass. I have never been able to determine what they were off of as I cannot find any with a manufacturer's number on them.
For the headlights, yes, any 7" bulb will work. In another thread I suggested an upgrade kit from the Bus Depot for a pair of EU H4 bulbs and harness which replaces the sealed beams traditionally used. Much better beam pattern in my opinion, and a bit brighter as well.