The 260(2.6litre) and the 300(3.0litre) are both M103 motors. The former simply has a smaller bore. The differences you'll encounter will have more to do with how the water pump is secured. Some use a combination of allen-head and hex-head. Benzmac has had alot to say about this subject. There is a tool made by Snap-On and if my memory is correct, it's called FLX-13 or FLX13. You might want to do a search on this tool number.
1991 300-SEL - Model 126
M103 - SOHC
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