No, the "PY" is simply another serial number prefix.
Starting in 1968-69 Colt began using letters with numbers as serials.
The "PY" range was made after 1985.
The publicly available Colt numbers end with 1985, so if you have a Colt made after that, the only way to get a production date is to call Colt.
Also, proofhouse.com IS NOT "Colt's website", it's a private site that posted the Colt numbers up to 1978.