Sunn amplifier dating
Note that other firms (as well as the crew in-house) did some wiring for Hiwatt at various times throught the company history, because Joyce would only produce a limited amount of chassis each month (20...40; accounts vary).
An important item to remember is that the date that the wiring crew put on the amplifiers was the date that they completed wiring the chassis for Hiwatt, not the date of final manufacture of that unit.
In the 1960s, musicians began to experiment with distortion by overdriving guitar amps.
All units manufactured by Hylight Electronics from 1966 to 1981 had a serial number plate like the ones in the pictures below attached to the metal chassis of the amplifier.
All of these plates bear the marking All units manufactured by Hiwatt Biacrown from late 1981 to 1984 had a serial number plate like the one in the picture below attached to the metal chassis of the amplifier.
I have not yet managed to collect enough information on the later models carrying the Hiwatt brand and manufactured by Turntone or Sterling, e.g. The serial numbers of these newer models often fall within the ranges listed below, but these were manufactured in the mid-80s and later.
Turntone started the "L" series of amps in 1985 or so, and these have no manufacturers brand on the serial plate, but say "Made in England." Amps made by Sterling Imports were made in California, USA, and have that brand name on their serial plate.