I looked this up on Wikipedia and found that indeed there are two sizes. This is what I found
"12 volt cigar lighter receptacle and plug, size A - Receptacle inside diameter: 20.93 mm to 21.01 mm (median 20.97 mm)
- Plug body diameter: 20.73 mm to 20.88 mm (median 20.805 mm)
- Most often used in American automobiles.
12 volt cigar lighter receptacle and plug, size B - Receptacle inside diameter: 21.41 mm to 21.51 mm (median 21.455 mm)
- Plug body diameter: 21.13 mm to 21.33 mm (median 21.18 mm)
- Most often used in European automobiles, and sometimes as a second socket in American automobiles expressly for DC power connections."
That answers the question of why two sizes. I am still trying to figure out if an adapter is made or not. The "Size B" receptacles are useless the way they are.
Tom