Pierre,
Here's how I understand the gas caps & tanks.
1. Both early & later 280sl use non-vented gas caps, even though they may have different expansion tanks. Yes some people drill a hole to make a vent, but this is usually if they have an issue with vacuum build-up due to expansion tank or vent line blockage.
2. The later cars (70/71) had the ONE KEY system. Same key for door, fuel & ignition. The earlier cars had separate keys.
3. There are TWO kinds of Mercedes gas caps that came with these cars. The "One Key" cars came with a cap that had a Key-Path that fit the ignition key. This cap has a key-slot that is centered. The cars that were not the one-key system, came with a cap that accepted a different key, not the ignition key, and the key slot is off to one side, as in the link you posted.
4. You can use either cap on any pagoda, but I don't think you can convert a non-ignition type key slot to accept the ignition key (if your car is a one-key car). You would just have a separate key for the fuel cap.
The classic center, from what I recall, has the same version as you posted, and maybe for less cost. Two or three years ago I know they also had the One-Key cap available at about $450 US$. I think the new production is only for the multiple key type but am not 100% sure about this.
This link shows the one-key cap (no longer avail at Auth CL), you will see the slot is in the center vs the one you posted.
http://www.authenticclassics.com/Late-280SL-Chrome-Gas-Cap-Plastic-Key-Head-p/auth-002470.htmHope this long explanation help,