I pulled a NOS Sangamo 500X203U006AB2B 20 000uF 6VDC capacitor out of storage, while looking for one of it's 16V rated cousins of similar vintage. Made in the US of A in 1975, week 34, so made some time between 17 and 23 August 1975.
Original testing ( on the $20 transistor checker) were 32.82mF, ESR 0.07R and loss factor 4.3% on 20 April. Then it was charged ( via a resistor) to 6.3V and left reforming overnight. Next morning disconnected, and left to self discharge. After a day it was at 5.3V, after a week it was at 3V9, and last night it was at 2V3 3 weeks later. Connected a resistor to discharge it, and this morning tested again.
Results are 33.03mF, ESR 0.03R and loss still 4.3%. So, basically after 40 years of sitting in storage, the unit is still within it's specs ( at least what I could find online for similar caps made by Sangamo at that time) and still is functional. Capacitance is within spec ( +100%/-50% for that era is typical) and it was holding charge quite well for the style, the 2V3 had been stable for over a week and only dropping very slowly.
