As is well known, the potentiometers used by Vox in many of its amplifiers (and certain guitars) were made by "Egen Electric Limited", a subsiduary of the Ekco group based on Canvey Island. Egen also supplied pots to Selmer and other companies.
For the time being, some preliminary notes:
The Egen Electric works on Canvey Island, c. 1970. Photo from this page.
Standard Egen packaging.
Below, examples of potentiometers from 1964, 1965 and 1967. Distinctive features are the flat bottom; the solid metal shaft; the two small rivets on top of the terminal board; the two lugs extending from the body, one flat, the other upright, each pierced with a hole; and the notch in the body of the pot underneath the central terminal.
"EGEN" is generally stamped on the top of the body close to the shaft.
The 500K volume pot from an early AC80/100 removed for cleaning. Date code is "AL" = January 1964.
The three pics above, a treble pot from an AC80/100 refurbished by Vox in mid 1965. The pot is a 1 Meg LIN (linear rather than logarithmic taper), date code "DM" = April 1965.
Early 1965 on
By early 1965, the volume and tone pots in AC100s came with stamped part numbers - 66181 for the former, and 66318 for the latter. Below, details from AC100 serial number 531:
Volume pot, 500K, part no. 66181, date code "AM" = January 1965.
Tone pot, 1 Meg, part no. 66318, date code "DM" = April 1965.
Egen potentiometers with printed part numbers are first seen in AC50s from early 1966 (amps with serial numbers in the 5000s). Volume pots (500K) were no. 66181; tone pots (1 Meg) no. 66520.
It is not entirely clear at present why the 1Meg pot for the AC50 should be no. 66520, and the one for the AC100 no. 66318 - they were identical as physical objects.
A 1 Meg tone pot from an AC100 with a serial number in the 1900s - date code "EN" = May 1966.
Details relevant to Vox AC100 mark 2s, 1965-1968, are being gathered together here.