found this that backs up my last answer:
Casting Numbers
Most of the cast aluminum and forged steel parts have a raised “casting number” cast/forged into them. This number has the same format XXXXX-YY as a part number, but you will never see this part number in a Spare Parts Catalog. The reason - it is an factory internal use only part number. Why would there be such an animal?
Harley outsourced aluminum castings and steel forgings. Harley did extensive machining and fabrication, but the castings and forgings required specialized manufacturing capabilities that were cheaper to buy than to build. For inventory reasons, Harley had the outside supplier cast/forge an internal use only part number into the “raw material” part.
Most castings required machining before becoming an actual part. The raw material Footrest support (left), which has casting number 50950-52, was drilled and tapped to become part number 50951-52.
Many "parts" are built from "sub-parts", so the casting number does not identify the part. A K-model Saddle bar (“seat tee”) is a good example. The 1952-1953 Saddle bar (Part# 51902-52) uses the outsourced front section (casting #51910-52) with an in-house made rear section. The 1954-later Saddle bar (Part# 51902-54) uses the same front section, but with a different rear section. To confuse matters, around 1959, the front section changed slightly (now casting# 51901-52A), but the Saddle bar's Part# 51902-54 did not change.
So casting numbers do not identify parts. But they can be important clues to the version, or production date, of a part.
