As I recall, serial numbers started off as 31-123 etc, detailing the model number and then sequential constructor's number (c/n).
This system was fine when Piper produced only the PA-31-310 and 300 (yes, there were a few normally aspirated Navajos built), however when the aircraft was stretched into the Chieftain, a system was introduced to differentiate between various sub-types.
In the early 70's (about 74 I think, but not sure), Piper adopted a system (for the PA-31's at least) where they used a longer constructor's number, the 2-digit prefix of which reflected the model number (31 being PA-31), then the dash (-), then the next following 2 digits reflected the year in which the aircraft was built; and then the next 2 digits identified what model Navajo (either 52 for the Chieftain , or 12 for the C/R or plain vanilla Navajo). For example 31-8152030 was obviously built in 1981, is a Chieftain, and the 30th example produced in that year.
Hope this makes some sense.