Bob Brown, the "Bob" of Bob's Services, is a founding member of the organization of antique car restorers and collectors that was instrumental in the creation of Historic Vehicle plates. (Take a look at Bob's collection).
Historic plates were first issued in 1966.
The 1968 plates looked like this:
The plates changed to a yellow base in 1970. Part of the early intent was to display the vehicle's year on the plate (see 23 and 37 below, both of which are on cars from those years.)
These numbers were soon exhausted, and regular numbers began to be used.
The original run of Historic Vehicle plates are stored with an HV prefix in the DMV database. This prefix does not appear on the HV plates. The prefix prevents the early plates from colliding with the legislators' plates.
After the HV run was exhausted, the prefixes began to appear on the plates.
The appearance of HX plates occurred relatively recently, so there were none on display in Bob's collection when I saw it.
Historic Vehicle plates do not require registration renewal stickers.
HW589 belongs to Chris Hubble, who first introduced me to Bob (for which I am forever indebted.)
Some of the more recent historic plates are using the newer, printed-on font.
... but they've now reverted to the 1980's-era partially embossed format: