The spec for a 1975 115.951 US version is 10 degrees BTDC at idle(800rpm)without vacuum. As a matter of fact...it's the same for any version of a 115.951 engine.
Mercedes-Benz makes some of the best literature there is. One thought would be to get a TDM(technical data manual)for the year of your car. All it has is specs. The shop manual or CD is good for DYI stuff, but sometimes lacks the information on specs.