These plates were issued in 1968 and were good for 6 years, until 1974.

These plates were issued in 1974, when the yellow ones expired.
These expired in 1980 and had at least 2 die types.

All-numeric plates issued with 1988, 1992, and 1993 expirations.
Plates in the 600- and 700- series were issued in 1980 to expire in 1988.
Plates in the 800- and 900- series were issued in 1984 to expire in 1992.
Plates in the late 900- series (??) expired in 1993.

Alpha prefixes started in 1980 when 12-year plates were issued to expire in 1992.
The prefix indicated the year of expiry:
P=92, Q=93, N=94, S=95, T=96, M=97, R=98, U=99, V=2000, W=01, X=02, Y=03, Z=04, A=05.
The last plates with wide dies (A series) were issued in 1993 to exipre in 2005.

In 1993, narrow dies were adopted.
The numbering continued where it left off, and 2 new prefixes were added:
P=2005 (used because the A series was now used on regular trailer plates), and Q=2006.

When the letter-prefix series reached 99999, a new 7-digit series was issued.
The plate number started with the year of expiration, i.e., 97 10000.
The last of these plates (and the last of the red LTT TRAILER bases) was issued in 2001 to expire in 2013.

In 1992, a new series was issued with 20-year expirations, and a SEMI-PERMANENT legend.
In 1993, these switched to narrow dies like the LTT plates.
After the expire-2014 plates, they switched to a numeric prefix.

Starting in 2002, as the LTT plates were phased out, SEMI-PERMANENT plates were issued for varying registration periods,
the longest being 25 years.
Some of the initial 25-year plates bear 12-31 expiration dates instead of 2-28.   Some years have plates for both dates.
When a series is exhausted, a letter suffix is used, i.e. 05 1000A
Starting with the expire-2028 plates, the series starts at 28 1000Z.   After 9999Z, they went to 28 1000Y, etc.