No need to argue over this. The proofs look genuine-enough Belgian proofs for me, and I have seen several pistols like this on the market here in Germany. As already pointed out - nothing to do with tigers, just a cheaper alternative to a real revolver for situations where one shot is not enough.
And the best candidate I can find for the chamber is....
... the 11.3x36R Gasser-Montenegro. A very hefty pistol cartridge indeed - 44 to 47mm OAL depending on the bullet and case version - longer than a 45 Colt, and not something which you would like to use for a target shooting competition. But possibly the best "man-stopper" of its vintage. An awful lot of "oomph" in that case! After repelling the assailant you probably needed a visit to the physio to have your wrist treated.
Anything longer than the Gasser would seem to be a carbine cartridge*, and hardly believable for a handgun - although I once saw a revolver chambered for the Snider cartridge!
However, the Gasser case has a rim diameter of about 14.2mm. Dttuner, could you please use your calipers to measure the INSIDE diameter of the rim recess, to see if a 14.2mm rim would fit?
*In fact, the long Gasser has similar dimensions to the cartridge for the M1867 Werndl carbine. Surely enough for anybody's wrist?**
** In 1882 it was replaced by a shorter version, and the remaining army stocks were sold to Montenegro. Hence the commonly used name.