I bought a high voltage insulation tester to 'weed' out condensers , before I build a pukka JIS spec. tester. (see JIS spec as detailed in 1966 CB450K0 Black Bomber Shop manual)
Interestingly, I took a dual capacitor from a Honda twin and a NOS similar one.
Testing the capacitor at elevated temperature, akin to what you get in service (i.e 80 °C in the JIS test, soaking at that temp for 30 minutes), the capacitance was bang on the correct value given by Honda on both the old and the NOS parts.
Also testing the the insulation firstly at 250v, then 500v on a single 'static' test, the insulation was fine on both the used one (Genuine part .. taken from a 1973 bike) and on the NOS one. I also tested a NOS electronic component 'wire ended' one of similar capacitance and voltage spec. The 1960's JIS test was 700 volts AC @50 /60 cycles/second)
However, there is a 'continuous' test mode on the new tester I bought and when tested on this function (it repeatedly applies the voltage albeit at a low frequency rate), the used one started displaying a value of a couple of megOhms leakage, whereas the NOS part and the 'reference' capacitor didn't.
So I guess the used condenser is leaky, dynamically and explains why the owner of the bike was reporting excessive arcing at the points, even though on a static test the capacitor tested out fine.
My next 'experiment' is to dig out a brand new Daiichi pattern one and put it through it's paces. This brand has a very bad press but is the pattern part sold by lot's of reputable parts suppliers including David Silvers.
UPDATE : I found a newish looking points/condenser assembly in my stash of parts and it looks like the points and condensers are all Daiichi. Stamped 'Made in Japan'
I tested the condensers both statically and continuously and perfect results. All of the NOS Denso condensers I have tested so far are OK too. The only used one I have tested so far turned out to be leaky but it's 48 years old, so understandable I suppose.
I will go through my entire stock of used and NOS condensers and see how they all fare.