Date registered: Oct 2012
Vehicle: 1968 250S, 1991 300se, 1998 SL500, 2x 1977 450SEL 6.9. Past MB's; 300SD, 300E, 300TE, 190E, ML420
Mentioned: 5 Post(s)
Quoted: 405 Post(s)
This question comes up a lot, and as you know a search really never answers the question.
I think most cars originally came with the air pump, and the tensioner was on there. If the air pump is removed then I don't really know the answer.
I would suggest looking carefully to see it there is and adjuster hidden away somewhere, or the other answer most posted is cut the old off and then force the new one on by turning the engine by hand to pull the new belt on.
Does your pump have and adjuster on it? If so please post a picture so future can see the solution. If you have a tensioner, then I would get a belt to fit.
EPC lists a 12.5 x 750, which would sit a little deeper and perhaps fit.