In NDH there were 3 military prisons in Zagreb , Osijek and Sarajevo. They were called "Posadni zatvor" ,meaning "Garrison prison" , military personnel charged with criminal charges was detained there. I dont known a single source of information about them, even the best book about Domobranstvo by Nikica Barić doesn't mention them.
Preserved mail to and from these prisons is super rare.
22.04.1942 parcel card for food package mailed from Zagreb to Posadni zatvor Sarajevo in Bosnia. Sender writes adress as 'Vojni posadni zatvor' meaning 'Military Garrison prison'. Prison cancel at the back of the card reads only 'Posadni zatvor / Sarajevo' , see enlarged detail picture. Prof.Ercegović certificate from 1984.
14.10.1944 card mailed locally from Posadni zatvor in Zagreb . Sender writes his adress as 'Domobranski posadni zatvor' located in Nova ves 18, Zagreb, while official prison cancel reads 'Posadni zatvor oružanih snaga' - 'Garrison prison of armed forces' see enlarged scan.