1. Pre-Memelgebiet stamp
99% of the regular Reich stamp that has been issued before 1920 could have been used in the Memel region. When I search German stamps in an internet auction, I usually ignore post-1920 Reich stamps up to 1939 to go faster. However, this does not mean that there is absolutely no way to find any postmark of the Memel area on a Reich stamp between 1920 and 1939 (for example, railway postmark Insterburg-Memel used in Tilsit could be on a post-1920 Reich stamp).
Pre-Reich stamps or also called Old German states' stamps were used in Memel region as well but only the stamps of Prussia and the North German Confederation.
|Memel postmark of 1886 on a Reich stamp|
These areas occupied by the Germans during World War I basically used German stamps overprinted "Postgebiet ob. Ost" but in some cases, regular Reich stamps were used mainly by the private postal delivery services. It is very difficult to find. Believe me...
3. Re-Annexation of Memel region
Nazi Germany re-annexed Memel region on March 1939 and used regular Reich stamps until the Soviet armies took the region in 1945. I would not say "frequently found" but those are relatively easy to find.
Ostland had its own overprinted stamp but regular Reich stamps were also valid through out the territory. As a result of the mass-production of Ostland stamps, the use of regular Reich stamps were limited.
|Postmark of Schaulen (Šiauliai, Lithuania) on regular Reich stamps|
|Schaulen postmark used for precancellation or also for parcel|
Those were the 4 cases where regular Reich stamps were used in the present territory of Lithuania for postal use. If I'm missing any cases, please feel free to comment on this post.