[djpateman]

That is a casting #. A casting # is essentially a part # for that particular piece. These numbers are raised characters and are part of the mold that makes the part.

If you want a serial #, then you need to look for numbers that are stamped into the part; that is to say they are recessed below the surface of the metal. If you are talking of 911 engine serial #'s, then look to the right of the fan on the supporting structure that is part of the engine case; also look for the type # that is located just back of the serial #. Most serial #'s start with 6 and have stars on both ends. The type # will be 901/xx, 911/xx or 930/xx in most cases.

look at this thread:

http://forums.pelicanparts.com/porsche-911-technical-forum/100507-third-number-engine-case-2.html