Brad;
Pull the chassis out of the cabinet and check the potentiometer code. On the back of the pots (volume and tone controls) you will see several numbers stamped into the back. You will see three digits followed by a hyphen, followed by four digits. The first three digits are the EIA manufacturers code which identifies the maker (Stackpole, CTS, etc.). The last four digits identifies the year and week of manufacture. For example, if the last four digits were 6952 then the pot was made in the 52nd week of 1969. Check all the pots in your amp, since some might have been replaced. Also be aware that some amp manufacturers purchased large lots of pots in advance, so the pot date can be one or more years ahead of the amp's date. Also check old Sunn catalogs for features. The 100S originally came with only volume, treble, bass and contour controls, up to about 1967. Then they added vibrato and reverb in 1968. Also check the grille cloth. Around 1968 they changed from the early, fine pattern to a larger rectangular pattern cloth.
Good Luck!
Ryan
