As you’ve said,it’s not just a crack,the frame has snapped.
Never seen that before,not in that spot.
Feal sure you have investigated to a degree,but why do you think it’s broken like that?
I’ve seen headstocks bent with buckled plates,fracturing due to internal corrosion .
Think I’d chuck it in a skip.
No available s/h frames ?
It was a bit of a shock to find it, I can tell you - adding the leaking tank to it and i felt like throwing the lot in a skip. Not sure why the frame cracked, but it looks like a brittle fracture, not like it started with a crack and progressed. I guess its due to the RS fairing adding weight and forces into that area - the fairing main bracketry hangs around this area ...I would imagine at some point it had gone down the road - i found a 1" scar on the crashbar, but nothing on the fairings. I suppose a small force from the crash bars would put a side force into the down tube and then after more fatigue it just cracked through.
The gu doing the welding really knew what he was doing - welding a tin foil tank up with existing repairs is a fantastic job...the frame repair was very easy by comparison with a full pen fillet all round.