The first three digits are the manufacturer’s code. The fourth digit is the year of manufacture and the last two numbers will be the week of manufacture.So if the number on your potentiometer is 304728 it is decoded like this: 304 is the Stackpole manufacturer’s number, 7 means 1967 and 28 is the 28 week of 1967.Note that speakers also use the same type of date code.Always remember that pots, speakers and transformers may have been changed!!This web-article details most of the various members of the NC-100 family and it became an extensive family (over sixty different variations) that grew as National up-graded and improved the receiver over its 13 years of production history.- Henry Rogers - March 19, 2012: The "MOVING COIL" NC-100 Receiver from 1936 (sn 334-D) showing the original "art deco" design of the aluminum overlay front panel.If three digits are present, the first digit would refer the units of the year i.e. If four digits are present, the first two digits refer to the year i.e. The last two digits would refer to the week of the year i.e.("26" would mean the 26th week or approximately June).
The Centralab cans have two turn stop indents; the CTS have one.
There are other methods used to date "Silverface" amps, you will need to get the date codes from your speaker frame, tube chart or pots.
Pots and speakers are usually stamped with a 6 or 7 digit code that can be dated as follows: The first 3 are the manufacturers code 137xxx = CTS 304xxx = Stackpole 140xxx = Clarostat 134xxx = Centralab 381xxx = Bourns 106xxx = Allen-Bradley Speaker Manufacturers codes 67xxx = Eminence 117xxx = Credence 137xxx = CTS 220xxx = Jensen 270xxx = Quam 285xxx = Rola 328xxx = Utah 391xxx = Altec/Lansing 433xxx = Cleveland 465xxx = Oxford 649xxx = EV 1098xxx = Pyle The next number(s) is (are) the year.
If you are unsure if the digit denoting the year is in which decade (1957, 1967, etc.) then try to determine this by the amplifier it is in.
Tweed amps were made from the very early 1950’s through approximately 1960, blonde and brown amps were introduced around 1961 through 1965, blackface amps with black tolex started around 1964 through 1967 and silverface amps took over in 1968.