Don’t get ya cynic-al if indeed he does have a hanger and cold sets the stays the the hanger will be out, he might get away with it but the hanger will be out.
Stays are welded to the BB shell, the drop outs are parallel. Spread the stays and the drop outs will move out of alignment, they’ll flare out. Never found it an issue on frames I’ve done with vertical drop outs, but ones with the early long horizontal drop outs have been ‘awkward’ But again keep it five or six speed friction and no problem.
Those Falcon drop outs don’t have IIRC a screw adjuster to set the wheels position, that combined with drop outs that are now not parallel and modern STI’s and you’d be lucky to get good shifting.
I’ve never used any proper tools, just threaded rod with nuts and penny washers using a spanner to open up them as evenly as possibly. And just reset drop outs in the workshop using eye and line 😕
I had a 531 frame with a 126 spacing and it would take a 130 though a wheel change could be a bit of a faff.
I’m sure when I had my last frame with 126mm ends built, the builder said the shape of the ‘short’ ends would allow the use of the new fangled 130mm hubs, something to do with a flared mouth?
Might have dreamt that.