1970 side marker bulb?

I have a few 1970 mustangs and cougars, so when replacing the rear side markers of a Mach I the bulb, and subsequently the socket, were a bayonet style versus the standard twist lock that takes a 161 bulb. Interesting enough there was an adapter plate allowing the bayonet plug to ‘snap’ in.

My cougar has the 161 style plug that twists in, so my question is did Ford wire some side markers with different plugs/bulbs, or has someone messed with the wiring?

I have found both types of side marker lights on 70’s. I believe the twist type were the earlier version.