Straßenbau Bataillon 521
Aufstellung 04. August 1939 als Übungs-Bataillon;
wurde 26. August 1939 mobil im Armee-Paket "W" = Wien zu 4 Kompanien;
Januar 1943 in Stalingrad vernichtet.
source:
https://forum.ww2.ru/index.php?showtopic=1367577
I can find two other listings for items relating to this unit, feldpost from two different men from 2. Komp.
http://www.stampcircuit.com/stamp-Auctio…ost-2-weltkrieg
https://www.ebay.de/itm/173284465672?rmvSB=true
If the unit was attached to the 16th Panzer Division, there is a unit history from 1958 (later reprinted) and a "Bildband".
If the Vermisstenbildlisten database ever comes back online, you should be able to use the Feldpostnummer for the men from 2. Komp. to find missing men from the whole unit.