The part number stamped on the muffler seems to indicate it was originally fitted on a 202 chassis, and was likely modified to fit the W124 E420. Remember this will have to be welded on, and may require sacrificing your existing muffler to make the swap. I don't know if it would restrict flow enough to reduce power, I doubt it, but it's a possibility. Price seems fairly high for a modified muffler, IMO. At a minimum I would request photos of the entire muffler, showing the other end with the inlet pipe.
If you just want more rumble, consider an aftermarket Magnaflow or something, similar to what Rik (2phast) did on his 500E; along with a Timevalve Y-pipe. Or, shell out for the 036-specific Fox exhaust that JC220 (?) and Maui both installed.