Found a hole!

[Bike Hoarder]

But... its just a pin hole, structurally comprised, still okay for some light seafront/towpath duty?

 

[bikeworkshop]

Interestingly, rust seems attracted to a single weak point - my petrol tank had several rust holes, but once id cut it open to look, the rest of the interior was almost perfect.

Consequently you could drill the hole back to good metal, probably just a few mm, braze in a tiny patch, treat the interior of the stay to a waxy rustproofing, and ride another 30 years.
Probably...

 

[Bike Hoarder]

Unless of course you can hear the flakes of rust rattling around in the tubes...

 

[legrandefromage]

It's inside out so, funched, farped, ducking knackered, etc etc

Rides it till it squeals

 

[Tootyred]

Cheap phone endoscope....

 

[Tootyred]

That's the "what 3 words" of the nearest scrap metal merchants btw.