Chris_ATL,
The following is from the technical manual:
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The engine number is located on the flat machined area below the spark plugs towards the rear of the engine on the driver’s side. If the machined area is there, but no numbers are stamped on it, this means you have a factory replacement engine. There may also be a small metal plate riveted onto this spot, with the words “Mercedes-Benz”, “Original Tauschaggregat” and the engine number included. Replacement engines were sometimes stamped by the Mercedes dealer with the original engine number using an appropriate die punch numbers set.
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If the engine is an original 280SL engine, the number should start with 130 983 ....
It might be helpful to read the whole section on the engine.
You seem to have full access to the car. Do you by any chance have access to any documentation on the car from the seller such as the data card? See the section in the technical manual on data cards. If you have the data card or a copy of it, which includes the VIN, it will also show the original engine number as well as a lot of other useful information.
As advised by Oz in another thread, the extent of rust may be the single most important aspect as to restorability.
