I have a white 4001 I've owned for over 30 years, which I have always considered a "late 70's" but the lettering has never been fully legible due to rusting.
It used to be better than it is now, & perhaps could have been JK or PK [there's an odd ding left of the lettering making it look possibly a J, but I thought P was more likely] It's pretty much impossible to see it now, it's got worse over the years. The number is still clear - 7474.

I'd heard there might be identifying dates on the pots, pencilled, but this appears not to be the case. The pots have what appear to be serial numbers - I can only clearly see on two of them 1377605 & 1377606

On the body, in unpainted but varnished gaps; under the front pickup is a simple + sign in pencil & under the pots what looks to be
36
4001-L-X
M

Is that enough info to figure out that the serial number was?

Pictures [it will only let me post three] - higher rez available if needed.

On the pots, the 137 identifies the manufacturer. The next four digits indicate year and week the pots were made, so the 5th and 6th weeks of 1976. That would correspond to a P as the first letter in the serial number, and if the second is actually a K, then PK would indicate November 1976.

_________________
--Bob Farace

2015 4003 Fireglo
1992 4003FL Fireglo
1983 4003 Walnut