thank you mehul, this is indeed a rare on, chambeed for a 500express cartridge with a 86mm = 3 3/8"long case.
first one I ever see and a little bit of mystery. I believe this was only made from collath and started as the latetest update for tha 500bpe around 1900 but was then load only with nitro. the 1914 collath catalog shows the 423 grain softpoint with 4 gramm Rottweil powder No.5 when the 10,75 with the 350 grain bullet have 4,2gramm of this powder.
have you got the pics from the old catalogs mehul, please post it here again if possible.
Designation for this cartridge is 500x86, thats not realy typical for a german cartridge, it have to called 12,7x86R 500 Exp.
Teschner &Collath was a allways a firm , going owns ways.
this "Exenter Verschluß" action starting around 1860 as a needle fire gun and is much older than other break breech action except the lefaucheux underlever