Metal detecting find identification HELP

[sgaitandjiev]

Hey group im from Bulgaria. I found this thing a while back while metal detecting in an old ww1 era fort in Bulgaria. I ususally find things from ww1 and ww2. The diameter is arround 72mm if i can remember correctly. The amount of corrosion points out that its from that era. I mainly find german rifle casings and all sorts of parts from ww1 german grenades. Also a lot of stuff from the granatenwerfer 16(priest mortar). So any ideas on what this thing is?