8.1914 K.u.K feldpost to Osijek . Mailed from 3.battalion 28. Infantry regiment of Croatian Landsturm ( 28.Pučko-ustaška pukovnija). Regiment was part of 104.Landsturm brigade that fought in Serbian campaign. Brigade was exclusively Croatian made of 4 Croatian Landsturm Regiments.
Card was canceled with Tarnstempel : 9132 in Square box. Tarnstempels or "Dumb" (tarn=hidden) cancel were originally used to cancel Postal money orders , at the beginning of war they were used instead of usual postal cancels to conceal location of mailing.
They were mostly used in Galicia in 8-11.1914 and in Tirol 6-10.1915 , usage in other areas is very rare.
This card was posted from Serbian front and this is the first usage of Tarnstempel i have seen on the feldpost card mailed to Croatia. Sender notes that his Feldpost number is Fp.9 (bojna pošta 9) that served 104.Landsturm brigade at that time.