Under the Treaty of Versailles, the Saar area was occupied jointly by the United Kingdom and France. In 1920, Britain and France established a nominally independent occupation government in an area separated from the previous Prussian Rhine province (main part) enlarged by two Bavarian districts (Homburg and St. Ingbert) ceded from the Palatinate.
This arrangement was sanctioned by a 15-year League of Nations mandate. During the post-World War I mandate, mail from Luxembourg to the Saar Protectorate was charged the UPU rate. Only after the Saar was returned to Germany did the German treaty rates again apply, beginning March 1, 1935.
Here's an example:
Vianden, transit Diekirch, to Dellingen, the Saar,
September 19, 1921
UPU 20g letter rate = 50c
UPU registry fee = 50c
The Souvenir I set (10c, 15c, & 25c) and the 50c Marie Adélaïde correctly pay the one-franc rate. If the cover had been mailed to Germany, the treaty rate in September 1921 for 20g letters (30c) would have applied plus a 50c registry fee.
Check the covers in your collection posted between 1920 and 1935 to the Saar Protectorate. Were they correctly rated?
In an earlier post, I showed covers paying the special post-World War II rates to the Saarland that were in effect from 1949 to 1958. You can find the post here.