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Filippo
Your guitar was most likely not made with hide glue in 1975.
Martin phased this out after moving from the old factory to the new one in 1964.
However no matter, some of these 70's Martins are highly regarded as of late.

As for replacing the cellulose acetate black pickguard I know of no source for this material.
Way back in 1975 the guard was coated with a solvent and literally fused to the wood of the top (take a look at the Sloane guitar repair book to see how this was done) then coated with lacquer when the the body was finished.
I have some old unused guards of this type but they most likely have also shrunken over the years as this black acetate was the worst for that problem.
I'm afraid that the Stew-Mac black (thin) stock is about the closest you'll get to the original.
Cut beveled and polished it's fairly close to the original look.

If I replace a black one, I seal the bare wood with a couple of coats of HHG. After it dries thoroughly, I scrape it flat and install the new black one with pressure sensitive adhesive.
I use 0.020" black rigid vinyl.
If it is fit carefully with the edges beveled and polished, the replacement is virtually undetectable.
Some of the old sheet acetate I have has shrunk and curled, but I am convinced that the main reason for shrinkage of the old pickguards is because solvent was applied to them. The solvent swells the plastic, and it shrinks once the solvent evaporates.
I have installed acetate and celluloid pickguards with epoxy, and they have not had any problems, even when lacquer was sprayed over them. The first one I did that way was in 1981.

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John

It's interesting that you use HHG to seal it. I haven't heard of that yet... Why not just shellack or lacquer? Perhaps because of the run out?

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"Preoccupation with an effect gives it power and enhances the error"
from "Your Owner's Manual" by Burt Hotchkiss.

The folks over at the UMGF have spent a lot of time researching the hide glue thing (ad nauseum) but I hadn't realized they had still used it for body assembly so late. Can't argue with evidence though!
As I mentioned, I still have some original old cut Martin pickguards in thin black acetate as well as a bottle of the original type solvent that was used to adhere them.
Not sure what the solvent is, I have a friend who is an organic chemist and he was going to analyze it for me. It's from an outfit called "FlexCraft" in New Jersey (still in business last I looked).
Certainly the lacquer being applied to one side only of these guards didn't help either isofar as shrinkage and curling.
I've had excelllent results as Todd mentions simply regluing these with carefully fitted cauls and fish glue. The problem is that you basically have to live with the shrinkage. This works well only up to a point depending on how severe the curl of the guard is and how much flexibily it has retained. I will always try to save an orignal guard (even a black one) if possible.

PM me Filippo if you want a couple of old unused ones to try. Perhaps you could epoxy them to the wood as John has done and over finish to replicate the original?

P.S. dealing with pickguards is another reason why I only build classicals these days <grin>

HHG is much easier, and it works fine. The water in the glue can also reverse some of the concave shape the top often takes when a rigidly attached pickguard shrinks.

I don't know any sources for black acetate. The acetate I have left is tortoise colored. It is the pickguard material Martin used on a few models in the late-1970's and early-1980's.

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John