I ran across some discussion of symbols used by BMW in the various catalogs and on-line fiche that we use to find parts. I'm sure this applies to most other models, but I see this all the time for our Airheads as we had many changes during the run of the bikes. I've also put this link in the Resources and Links thread in the forum.

http://bmwmotorcycletech.info/partsymbols.htm