[lugerholsterrepair]

Alf, Doug is correct. Synthetics were not developed to any extent untill the later years of WW2. Most all holsters and other leather items made by any country prior to 1945 were made with linen thread.
Linen is a plant fiber made from the Flax plant. It is twisted from smaller strands much like hemp rope. It is typically seen in 2-3-4-5-7- strand. German Luger holsters are usually made with the 3 or 4 strand. Oddly enough the later the holster the thicker the thread. 1940-43 holsters usually have very thick thread.
Yes, linen thread is available today. I get mine out of Ireland.
There are substitutes for it but they are inferior to the real thing for a number of reasons.

Jerry Burney