Below has been edited:
I replied last night without doing any research. Bad decision.
According to the document in the book 1980 is the first year for the new 17 digit VINs (previous VINs had 11 digits). As of 1980 the 10th digit became the year code. But that is wrong. The last GT5-S, 9562, built in 90, still had an 11 digit VIN.
I've never paid attention, or put my mind to the task of understanding the later date codes. It should be easy to do if a list of the known complete VINs were strung down a column, because a pattern of years and months will appear. I took a quick random look at some of the VINs in the 9000 series registry. Here they are.
THPN ?? 09001 … Born on 6/13/76 as per Van Ness.
THPN SJ 09016 … earliest whole 9000 series VIN in ProvaMo.
THPN SM 09047
THPN SP 09048
THPN SM 09050
THPN SB 09056
THPN SG 09070
(This year has the same month code sequence as either 1973 or 1974)
THPN TU 09094
THPN TU 09098
THPN TM 09101 ... approximately the first Embo coach (1978)
THPN TD 09103
THPN UJ 09127
THPN UP 09136
THPN UD 09137
THPN UD 09138
THPN UR 09142
THPN ?? 09150 … first GT5 as per Van Ness.
THPN VT 09159 … GT5 Turin show car, assembled 5/13/80 as per Olczyk.
THPN VU 09164
THPN VD 09173
THPN VR 09175
(Possibly the same month code sequence as 1974, where T = April ???)
THPN ZU 09203
THPN CA 09375 … first GT5-S assembled in 1984.
THPN FY 09495
THPN IU 09562 … last GT5-S assembled in 1990.
And here's a table with my assumptions about year codes, filling in the gaps. The month codes aren't complete … yet. This needs a bunch more complete VINs in the list in order to sort it out.