Jack:
Back in the 50's and 60's, when these cars first came out, you were lucky to get an outside mirror at all, let alone have one on BOTH sides of the car. Most of the 356's I remember seeing or riding in back then only had an outside mirror on the left side; some on the door, and sometimes on the fender. Either way, the mirror was designed and built for the left side of the car.
British cars, on the other hand, had those nifty bullet-shaped outside mirrors all over the place, mostly because if you left England for the mainland, all of the Continental crazies drove on the wrong side of the road (the RIGHT! for God sake!) so you needed an outside mirror on BOTH sides just in case (besides, the right side mirror was better to check out side-of-the-road chicks when travelling on the Continent).
Many years pass, and modern drivers have gotten used to having mirrors on BOTH sides of the car, mostly because other modern drivers have a strong tendency to pass other cars on the right (illegal in most states, but they do it anyway so car manufacturers put mirrors on both sides).
Long winded answer, but the 356 parts reproduction guys (Used to be Hans and Fritz, but they are now Yin and Yang) knew they already had a mirror stalk casting in stock, they got orders for both left and right versions of it so they just produced twice as many, put half in left side boxes and the other half in right side boxes.
There you go. You either have two Yin's or two Yang's, when what you really need is a Yin and a Yang...
Hope this helps.....gn
Oh, and that little mod with the clear suction cup? I wrote it. It really works, takes about 5 minutes to do on each side and makes the original problem go away. It makes the Yin on the right balance the Yang on the left.