Classtime 

I presented this problem in C&V

https://www.bikeforums.net/classic-vintage/1266850-mtb-grand-record-dropout-mystery.html

but maybe I’ll get more ideas over here.
This Campy dropout came to me slightly “opened up”. Do you think it is safe to close it a little so the faces are parallel? If so, how would you do it?

unterhausen

I would do that with only a little heartburn. You have to do it, it's not going to work well the way it is. I did this fairly regularly when I worked as a mechanic back in the '70s.

I have an adjustable wrench where the hole in the back fits over the hanger well enough that you can bend it with that. Or you can use a regular box end wrench. I know other people use other tools, but this has always worked for me. Dropouts are pretty ductile.

Trainbike

I second this as standard practice in bike shops. A related question: I worked with someone who when he had a drop out that was very bent, often a intergrated der hanger, he would bend it then heat it up to a dull red then let it cool. He claimed that this counter acted the metal fatigue. This makes common sense to me, but I've never felt confident enough in this to charge a customer for it, but have done this on my own for non critical applications. Does this make sense? And if so are the more detailed specs of how to heat and cool the metal?

unterhausen

I don't think it makes sense at all, actually. It may not hurt anything, other than the paint. But I think the paint is fairly important.

It's not really fatigue, it's plastic deformation. And leaving it with internal stresses makes it somewhat more immune to fatigue cracking. If there was any permanent plastic deformation that made some sections of the dropout thinner, it seems really unlikely that heating it red hot is going to change that back to the previous condition.

Fredo76

Annealing (heating then cooling slowly) softens the metal and removes brittleness. Heating and then cooling quickly is a hardening technique.

https://www.engineeringchoice.com/what-is-annealing/

Classtime 

Unterhousen’s technique worked. A box end wrench over the derailleur hanger gave me a lever to squeeze against the chainstay to close the gap. I have seen many pictures of broken or repaired horizontal dropouts that made me hesitant to try anything. It turned out to be no more dramatic than straightening out fork blades or seat and chain stays. The process buggered up the derailleur stop a bit but no biggie.