The rifle looks like a pre-WW2 "Original Wilhelm Brennneke, Leipzig" rifle. These famous rifles were all built for Brenneke by Schmidt & Habermann, Suhl, and have their small "esha" trademark hidden somewhere. Often esha is found under the magazine floorplate opening lever or under the barrel. The rear sight base with the serial number is an Schmidt & Habermann earmark too. But the bolt handle and front sight cover seem to be modern replacements. The photo showing the proofmarks is quite fuzzy, but seem to be BRU, indicating a pre-1940 rebarrel job. The "DINOX" scope by Bischoff, Hildesheim, is of prewar vintage .
The 6.7x64 caliber marking is unusual, but explainable. Most European cartridges got their name not from the bullet or groove diameter, but from the smaller bore/land diameter. So the popular "6.5 mm" cartridges use .264" = 6.7 mm bullets. Some countries or companies named such cartridges for their bullet diameters and did not count the thickness of the rim into the case length. I have seen some 1920s Czech 6.5x54 M-Sch cartridges labeled 6.7x53. BTW, "7 mm"s use .284" = 7.2 mm bullets, .303 British .311" ones, while .308 and .358 Win. are on the nose.
The 6.5x64 Brenneke cartridge was released indeed in the 1990s by the Brenneke, Langenhagen company as a new development and is not mentioned in any literature on the prewar Wilhelm Brenneke cartridge developments. But old Wilhelm B. was an avid experimenter. Besides his own better known cartridges (in chronological order) 8x72R, 8x64S, 8x65RS, 7x64, 7x65R, 9.3x64, 9.3x65R, 7x72R, he is also known for experimental wildcats like 8x80R, 8x66.5 and his "Brenneke Maximum" experiments on the .404 case, 7x64.5, 7x66 (copied by Gehmann postwar and named "7x66 vom Hofe Super Express"), 8x66, 9.3x66. So I can not exclude his experimenting with a 64 mm long case using 6.7mm = .264" bullets and having an experimental rifle marked more correctly 6.7x64 built by Schmidt & Habermann.
Leipzig was occupied by US Forces on April 18, 1945, and handed over to the Soviets on July 2, 1945. Plenty of time to "liberate" the contents of Wilhelm Brenneke's shop and home.