When my VW Rabbit PU horns rusted out, I replaced with W123 horns. Plenty loud now.
I have heard that the 600 horns of the 60's are only replaceable as a swap -- must have the broken part in hand at the dealer to get a new one. They are very special horns for a very special car and MB wants them that way.
1986 300E Anthracite + ECodes + MB Mileage Award