Pontiac Part Number & Casting Number Prefixes
Information from Pete McCarthy's "Pontiac Muscle Car Performance 1955-79" and POCI "Official Judging Manual", Appendix XVI
There is a correlation between part numbers (and casting numbers) that matches the years they were produced. The casting number on a part is usually (always?) close to the part number but often not the same (casting Number ≠ Part Number).
The following table lists the numbers from 1955 through 1962. Obviously, in many cases, an old part number remains the same during later years, because there was no need to change it. In addition, there is a certain crossover from year to year, as numbers assigned late in a given calendar year would be used for the following model year. Six-digit part numbers (and casting numbers) were used 1955 through 1961 with the first three digits identifying the year the part was first produced.
year - part numbers
1955 - 518xxx 519xxx 520xxx 521xxx
1956 - 522xxx
1957 - 523xxx 524xxx 525xxx 526xxx 527xxx
1958 - 528xxx 529xxx 530xxx
1959 - 531xxx
1960 - 535xxx 536xxx 537xxx
1961 - 538xxx 539xxx 540xxx 541xxx
1962 - 542xxx 543xxx 544xxx 545xxx 546xxx 547xxx 548xxx 9770xxx
1963 through 1970 used a 7 digit part number / casting code in the form 97xxxxx