Where's the vent hole? Often they are located about where the crack is, on the inside of the CS (out of our sight). One likely contributing factor is the chrome plating. It also sounds like you live on the coats.

Here's a possible scenario. Chroming fluids get inside the CS via the vent hole. Fluids are not fully rinsed out. Over time the remains of the fluids with the salty and humid air accelerate the internal rust.

Another not uncommon possibility is that the drop out was kind of over heated and the stay embrittled a bit. The crack looks to be close to where brass/bronze filler can stop at. This would be where stresses focus, at the boarder of a thick to thin section change.

Even without much environmental salt hydrogen embrittlement is a real issue when chroming steel.

What to do? If you try welding any chrome needs to be removed for the filler to wet out (and burning chrome fumes are really bad to inhale). If there's brass/bronze close enough by to melt when the weld it done that needs to also be kept unmelted as welding doesn't like molten brass/bronze in the bead. The stay's face will want to be cleaned and a shallow "V" ground in it to help the weld penetrate all the way in. Hopefully any corrosion that is up in the CS can be accessed then reduced/treated before any repairs are done. If you have a local welder who works with thin wall alloy steels and also is motivated to do good work on a bike I would try this first. because if it doesn't work out you can always go to plan B, a complete stay replacement.

All this assumes the shinny drop outs are chromed and not polished stainless steel. If they are SS then the weld is pretty much the same. The CS replacement would be slightly more challenging, though not by much. Andy


Assed thought- I suspect the inside of the CS has been beveled some to better clear the chain/cogs during wheel removals/installs. If this relief was done poorly that might also contribute to the crack being where it is.