For the longest time I figured since the only thing that seemed to differentiate the ammunition used by the 8,8cm Flak 18/36 and the 8,8cm KwK 36 L/56 was the primer (C/12 percussion for the Flak, C/22 electric for the KwK), there'd be no real way to tell if a particular round was specifically intended for one gun or the other- anyone can swap out a primer; but it turned out that the Tiger's gun actually used specific ammunition afterall- when I finally got a copy of its manual, quite clearly the diagrams showed the casings being labelled '8,8cm K.w.K. 36'.
I was extremely lucky to find one for sale a while later, just listed simply as an 88mm fired case that someone had picked up someplace in eastern Europe- but I recognized the vital marking and clearly original primer and knew it was really a Tiger round case. Even better it was clearly marked 'Pzgr. 39/1' on the base so it was an armour-piercing round! Clearly fired too because the crimps were pushed back out and the neck slightly flared (not the distortion commonly caused by removing a projectile manually).
This is one of my most favourite pieces because of its rarity and because it isn't just unissued stock, but was, presumably, actually carried and fired in action by a Tiger on the Eastern Front