Advice needed: Old Lindberg "Press-Cal" Decals

I'm afraid that there probably isn't much you can do with them. Back in the mid 60s to the mid 70s Letra Set produced a series of rub on "decals" and they probably produced the set you have for Lindberg. The initial issues were black and white (Luftwaffe as you might suspect) with the latter issues being multi colored. I loved em because they went on easily and looked like they had been painted on and there was never any problem with silvering like a standard decals. The biggest problem turned out that if you didn't use them within 6 months or so after purchasing them, they would either not stick to the model (most common) or portions of the film would stick to the model but other portions would not come loose from the backing sheet, tearing the decal. Letra set got out of the modeling decals but still makes rub on letter and number sets you can find in the craft sections of a lot of stores.

if you can, id say treat the "press-cal" things like a piece of paper and use white glue and then seal with dullcote or future. this is just a suggestion, im afriad i ahve no experience with the things

I have some old rub-on instrument decals that have the same problem. Since they have to go on a painted background, I paint gloss white and wait until it's just tacky, then rub them on (gently), with something like 95% success. Maybe you could try a clear coating, depending on your situation. Obviously, if there are any "spare" markings on the sheet, you can practice on a non-critical surface first.
Best of luck.