Just in case others may find this helpful . . .
I have acquired front and rear mudflaps for my 1997 SL500. The rears are like hens' teeth, being no longer available from MB; perhaps they'll come back, as discontinued parts sometimes do. The first rears sent by an independent supplier didn't fit, their corner fit being too angular. I then took a punt by changing the penultimate digit in the part number and putting it into Google. There was just one return from the whole world, from UK eBay, but I couldn't find that specific feedback post in the vendor's listing, so I wrote to the vendor. Glory be, it turned out that he had an unused pair of rears that wouldn't fit his 1992 car but will fit mine, so we've swapped.
So, the numbers are as follows:
Rear Q34012920 (slightly angular)
Rear Q34012940 (smooth curve)
Different again? Not sure.
I've read elsewhere that mudflaps can't be fitted to R129s with AMG body kit. I have a feeling that some of the special editions and the Silver Arrows may also be in that category (do they have AMG bolt-ons or their own variants?). Having looked at dozens of photos, I'm inclined to think that other post-1997 cars have the same bumper-end profiles as my car. Someone will no doubt confirm.