As you guessed there is a back story (isn't there always)
Here goes, and sorry for the long winded tale.
The original starter failed several months ago, and I took it into a local auto electric shop for rebuild. They said that this would be too difficult and would take months, so they sold me a modern unit that they swore was the exact replacement part. I fitted it, and it worked fine for several weeks, then started to run without meshing the ring gear, so couldn't start the car. I took it out and it was then that I noticed the metal shards, very fine, clearly from the ring gear. The ring gear was visible through the bell housing, and the teeth were all present perhaps rather worn but not too bad to my inexperienced eye. It seemed clear to me that the modern replacement was not a mechanical fit and I was very worried that the ring gear was toast.
The auto electric shop refunded me for the starter but refused to accept that it might have not been the right part. I subsequently found out that 1974 was a special year re the ring gear tooth number, and that not all parts suppliers recognized this fact. Not wanting to take any more risks, I bought a old recon unit from centerline, identical unit to the original starter. Fitted it in, and it worked fine again for a month or so (the car is only used once a week at most). Latterly I noticed that the starter would be a little slow to get going on turning over a cold engine, and also that the noise on starting would sometimes give a little intermittent "crash" sound as the starter was running. which I attributed to the engine firing but failing to pick up. There was never a grinding sound, and never a failure to mesh.
The most recent event was the complete failure of the starter this week, couple of cranks then it was dead, no click, nothing. The starter was issuing gey smoke, and the starter housing, not the solenoid, was piping hot.
I have sent the starter back to canterline since I worry about an internal short. However, the ring gear is worn and possibly further damaged by the episode with the modern starter.
Here's my concern: is it a reasonable bet to try again with the reconditioned unit if centerline can repair it, or am I going to keep destroying starters because the ring gear is worn? From my reading of the alfa BB, ring gear problems seem to mostly result in terrible noise and failure to mesh rather than starter destruction...
Thanks for the input everyone, I appreciate the time you have taken to think about this
TS