Hi, welcome to the forum!
Your first stamp is from the DDR Five-Year Plan series, the third issue (20 pfg on 24 pfg overprint (surcharge)) issued in Oct 1954 with reprints in the following years. Your example is the letterpress version, which I believe is more common than the offset version. In terms of rarety or scarcity, they are generally very common. One good thing about your example is that it looks like a postal usage, rather than the more common cancel-to-order (CTO). The Five-Year Plan series has a wide variety and is a fascinating series to build a collection. For more detail, the following link would be a good reference.
https://worldstampsproject.org/gdr-...tage-stamps/You also can find many topics on this Forum - try the "Search" button and look for "DDR Five Year Plan".
Your second stamp is from Germany (German Reich or German Empire), during the inflation era in the 1910s and 1920s. Your 2 Million on 200 Mark overprint is issued in 1923, and has a Michel catalogue number MiNr 309. Again, it is a very common stamp in general, but it also has a wide variety in shades, watermarks etc, and is again a fascinating series for collecting. For further details, I recommend you to refer the following site;
https://www.germanstamps.net/wmr-rp...923-inflop3/ Here again, there's a lot of dedicated topics on this Forum so use the "Search" button to seek for, example, "German Inflation".
I have spent my childhood in (West) Germany, where I started stamp collecting, and the Inflation series and DDR Five-Year Plan series did appeared very often in the stamp packet I bought there. I am still keeping them, and occasionally adding the missing ones - still enjoying what I've been doing 50+ year ago.
Regards,
Hironobu