Some early W210 Mercs came with 15" alloys or steels.
The part number for the steels should be A 210 400 0702. If you can't find a full set try the boot of an early W210 in a scrappy as it should have a 15" spare.
The standard alloys off of my W210 are 16", so I'd expect the steels to be the same
Perhaps they were 15's.
Basically stock W202 C250 steels don't fit W210 E300TD..
I'm sure the bigger diesel W210s had different front brakes, so the spare/steel for the E300TD would be 16" for clearance.
The 15" steels would only have been used on the early four cylinder, probably lower spec, W210 chassis cars.
The PN on my 7.5" wide 16" black steels is 2104000802. That's the PN for the steels for the V6 and V8 cars.
Confusingly, there's a part number for 16" black steels that apparently don't fit the V8 cars for whatever reason, I think to do with brake clearance again, and there seems to be yet another PN (which I don't have to hand, sorry, which is an 8" wide 16" black steel.
I have a set of the 8" rims that I've never gotten around to getting tyres on because I got offered a set of 7.5" rims with virtually brand new tyres (via this forum) about ten minutes after I bought the 8" wheels on Fleabay.
I didn't buy the Benz steel wheel studs, I got the same size from the VW dealer (Polo part) for a couple of quid each.
(I know that's outwith the scope of the original question but I thought I'd leave it here so that nobody has to repeat the various mistakes I made when swapping to black steels)