Look, I am not an EPC defender here, as I say - it has its dark sides. But they are rather because of difficulty to read it.
But I would be very careful about judging EPC is not correct because a friend of mine has a cover on his throttle body. Actually EPC is one of the key sources of information on what was fitted to which car. It is the electronic archives of Daimler-Benz, there is no other records than this and the paper booklets, that seem to be saying the same thing.
How can a cover get to the throttle housing:
1. Over the last 50 years there might have been a mechanic who, when he was supposed to give back the car to the owner saw this empty space and he thought he forgot to fit the cover - so he fitted it.
2. The mechanic said to an owner: "Look, someone left a hole here where in automatic there is a switch, I had one, so I screwed it on". The threads could have been there, as the throttle housing changed over time.
Or the owner looked at the throttle while picking the car from the shop and said: "Where the hell is my venturi shaft cover?"
3. The throttle housing has a different number, but maybe it is similar enough that somebody installed or mixed the 982/984 one.
4. Maybe the only difference in these throttles is the threaded holes and presence of the cover? Then if the factory was short of the 981, they would install 982/984 throttle.
5. The car you mentioned has the throttle replaced.
6. Somebody mixed the picture from the first post and it is not 250SL, but a 220SE standing next to it in the garage.
...
There may be a 100 reasons why this cover happened to be there.
You say there are no threads in the holes for potential screws. That suggests something.
I usually take the EPC info as one of the most reliable sources of this type of info. Paper part booklets may be better sometimes. But as I read below Alfred says it "is in manuals for 220SEb only".
To me it is quite clear, actually, because of what EPC says: 230s and 250s did not have the plate by design. They might have gotten it by accident.