As an addition to what has been discussed,
Kemet was also licensed to manufacture the exact same part.
The shelf life of super capacitors are rather limited and when it comes to NEC, the production date code even more dubious.
Since NEC has gone through so many changes, I do not believe they even know it and good luck finding that.
As an Example, if you were to follow their old numbering system, you can see the accuracy of date code "M"
In my experience, the majority of super caps pulled from PCB's are either dead or about to. In my days with Consumer Electronics, I used to see it very often across the product lines.
An Go-No-Go test is to utilize an adjustable power supply. While it is not exact, it is a good indicative.
Set it to zero and hook it up (watch polarity) and while monitoring the voltage, slowly advance it to cap's voltage rating (5.5 V in this case)---
no more.
Allow it to sit about a minute and turn the power supply
back to zero. (If using supply w/ analog meters, at this point you can turn the supply off)
A good cap will retain the charge and your supply is now a voltmeter as it will show the decaying charge.
Depending on the capacitance value and condition, the decay time varies.
As a rule of thumb, if the charge is held more than 30~45 seconds, it is probably a good cap.