Ah, we seem to share a love here. Good to know.
Different techniques were used: airbrush as well as pencils, ink pens, paint brushes, diluted white or light paint. I think you can still buy some of the retouche paints (Schminke -or Schmincke?- still sold them some years back). IIRC, one worked on rather contrasty prints of the original analog photographs. Afterwards a copy of the finished art work was made on a contrasty (lith?) film used for offset printing.
It took regular and thorough training to work on a retouche department. If you want to find out more about the tips and tricks of the trade, you do well to look for books on vintage advertising techniques, vintage (pre-press) graphic design and halftone printing techniques. In very early air brush manual you may find a chapter on photo retouching. It may be worthwhile to figure out how to do this without a computer, since it takes as long with the aid of software programs as it does by manual labor.
HTH, Norm