Looking at the diagram from the link in my #11 post, It has a condenser illustrated behind the breaker plate. The technology of that era was such that most capacitors were made using wax paper as the insulator for the wrapped foil internal to the part. Wax paper deteriorates over time to a point that it will no longer insulate as it should. I don't know if there is a company out there that either fabricates a new part or repairs an old part for these magnetos (see link below). I know in the case of the smaller Lucas K2R magnetos, folks substitute a modern capacitor of the same microfarad rating and back voltage absorption capacity as the original but these are a completely different design. This link better illustrates the DU series and also has a better parts break down.
https://oldcroak.com/du1-du2-du4-ad-nauseum-catalog-50/
There is a photo of an old armature in the link where a person can see the condenser housing just adjacent to the gear on the opposite end from the high tension slip ring.
An old magneto may make a spark on the bench with a bad condenser but it would likely break down at higher rpm. I'm not telling you it's bad but after so many years, it would be very likely that it has deteriorated. It would have to be tested with a capacitor tester to know for certain. You might check with this outfit (link).
http://www.mainelymagnetos.com/BoschParts.html
They list a rebuilt condenser for $30.