OK, let's look at the two types of serial numbers given for the PA-31 series, and we will dissect them into what each part means.
The serial number looks something like this: 31T-8275002
Most serial numbers are split up into 4 sections
(31T)-(82)(75)(002) 31T-8275002
31T - This is a PA-31 turbine series of aircraft, if there was no 'T' it would be a piston aircraft.
82 - This is the year the aircraft was registered and ready for sale from Piper, 1982
75 - This is what is called the serial number block, which denote what sub-model of the PA-31 series this is, PA-31T3 T-1040
002 - This tells which one this is out of the batch from the year it was registered and sold, 2nd one off the line.
The other serial numbers look something like this: 31T-5575001
Some serial numbers were changed removing the year of manufacture, and replacing it with plant codes, split up into 4 sections
(31T)-(55)(75)(001) 31T-5575001
31T - This is a PA-31 turbine series of aircraft, if there was no 'T' it would be a piston aircraft.
55 - This is the new designation for the plant of manufacture, 5 is a designation for Lock Haven
75 - This is what is called the serial number block, which denote what sub-model of the PA-31 series this is, PA-31T3 T-1040
001 - This tells which one this is out of the batch from the year it was registered and sold, 1st one off the line.