Undercarrige paint is primer
Body color was single stage (one component) enamel
You are asking about the chemical composition of the original paint. I dont think it matters what you use. Most concours cars do not use original paint systems, because of enviromental laws or the fact many of these paints are no longer manufactured. I would be surprised if you can buy single stage, acrylic enamel in Germany. I am pretty sure the original paint has lead in it and it is no longer available.
I think you are worrying too much about the exact type of paint. Pick a paint that you can have sprayed that looks good to you. No one is going to do a chemical analysis of your paint job.
This is like tryingto buy a light bulb. If you go the store and ask for a light bulb for your lamp, they are going to hand you one of theose squiggly things. It will still work.
I sprayed my car with original acrylic enamel and no one even knew. They all assumed it was a base coat, clear coat modern paint.