...interesting. Given a well fitted (snug slip fit) post that extends far down into the seat tube, what do you think will be the manner of eventual failure ?

I get that the stays do push up and a little forward, but if this were a high stress area, that would argue about ever slitting a seat tube/stay lug as a construction method in the first place. I think. Maybe. I could be 100% wrong on this. My best guess is that the initial cause for this was someone running a seat post in too small a diameter. Which is just my best guess, because it came with only a Tange bottom bracket and an Avocet headset.

No history, no provenance. Anyway, if I get an answer from Steve Rex, I'll post it in the thread. Like I said, I've never personally seen this mode of failure before.