The kind of paint Europeans used during the age of your bicycle and the kind Americans have used since the bike boom are not the same at all. When I was learning how to build frames at Ellis Briggs in Yorkshire in the 70's, our frame building room on the 2nd floor was next to the painting room. I learned a lot just watching them. They used stove enamels which had to be baked in big ovens once the paint had been sprayed. Most Americans started with Dupont Imron a high gloss epoxy type of paint in which an activator is added just before using so it hardens by chemical action instead of solvents evaporating. Of course polyurethane enamel (which is what Imron is) is made by a number of manufacturers including a company just down the road from me. There is also another type of paint we use too. This difference of paint types means touching up your frame so it looks like new is not a simple process and may not be what a typical American painter can do well.
I don't do as much painting as I used to, I primarily teach frame building and painting classes. What painting I usually do is for student frames. What pictures I take is typically just for documentation and to send to potential students to wow them to realize what they can do if they take my class. Besides that I make no effort to market my building and painting skills.
I would guess that dabbing in a little House of Kolor into a small damaged area would work okay. It is designed to be cleared over with urethane but that is a high gloss paint and is not compatible in looks with the old finish on the frame now. I'm also guessing you don't have big scratches so just letting it air dry would be an improvement over doing nothing and help prevent it from rusting.