Michael,
We replicated the length of the post to exactly like the original. On my 250 SL, the data card says the gas cap and door are the same. However, like you, I have a gas cap key that is different. I suspect 50 years along the way, a replacement gas cap and key came along, both YMOS. You could have the cap keyed to your door key by a locksmith or, do as I, and just keep the gas cap key in the center tray or glove compartment in a cheapo fob.
John