Which adds to the confusion as to where have the sparks gone???
Probably not very far, but it's going to be hunt the thimble to find out. A sudden total lack of sparks both sides, not a glimmer, speaks more to a low tension / contact breaker type problem than to a failure on the High Tension side (usually).
A few thoughts:
Low Tension side:
1. Have you tested for sparks with the screw-on end cover and kill button detached from the magneto? A short between the carbon brush and the cover, or where the wire attaches on the outside, will stop it working. (Make sure when testing that the blade that is supposed to have the button under it can't touch the head of the centre screw if it's out of position - that would short out the low tension side as the screw and the brass bit it passes through are 'live'.)
2. Get the cb unit off - one screw. With the points closed, do you see as near as makes no odds Zero ohms across them? If there's a high resistance the points aren't making good contact for whatever reason - oxidisation being a common one. Cleaning to squeaky clean will often resolve that.
And then with them open, do you see an Open Line? (No good testing with the cb on the mag as there won't be an Open Line when they are open - just the half ohm resistance of the low tension winding.) If there's continuity with them open and off the magneto with the centre screw shoved up the middle, then there's a problem - which is probably a broken insulator, the one the cb centre screw passes through, or the mica flat insulator that goes between the fixed point carrier and the cb backplate.
3. Put a meter between the cb centre screw (no need to put the whole cb unit back on) and measure the resistance between it and the mag body. It should be about half an ohm. If there is no reading, then either the electrical connection to the 'nut' inside the mag which the centre screw attaches to, or the earth connection from the coil up at the condenser end, is faulty.
Low tension windings themselves fail so infrequently it's a million to one against there being a problem with the winding itself.
High Tension side:
1. Remove HT leads and check for continuity from the brass washer under the acorn nut to the spark-plug end of the lead. Should be Zero ohms. Avoid using high resistance plug caps if possible, and don't use carbon leads. Copper-cored for choice. Not a likely reason for your problem as both sides wouldn't go funny on you at once, but to be checked all the same, especially if using those flexible rubber plug caps whose screw tends to go off centre when attaching to the HT leads.
2. Remove HT pick-ups and check the brushes are there, and in good condition. Check continuity from brushes to the bit the HT leads screw up against. A modest resistance of a few ohms would be pretty normal.
3. With the pick-ups removed, turn the mag until you can see (maybe using a small mirror) the brass segment on the slipring which the brushes run on through one of the pick-up windows. Put a meter on the brass segment, and the other end to the mag body, high resistance scale. You should see about 5000 ohms. If you don't, then there's a break in the HT continuity. The HT winding is earthed up at the condenser end in common with the low tension side - so an earth problem would probably affect both. Clean the slipring if it's mucky, using a wooden rod with a bit o' rag on and a solvent. Shouldn't be mucky unless you have fitted poor quality brushes since it was overhauled. Othe problems could include the connection of the coil to the slipring being dodgy, or that there is a fault on the coil or condenser. Highly unlikely given the excellent provenance of the thing but 'stuff happens'.
I'm guessing the problem is at the contact breaker somehow, but these things are capricious and the only way to proceed is 'one thing at a time'. If everything seems good per the above, then I think you need to get back to Andrew after he re-opens for further advice and suggestions!
