What is “K67”? It is AP and has something to do with bullet material?
My only WWI Spandau case is from December 1915 (your case is from August) and is stamped S67, not K67. S67 indicates the brass used to make the case contained 67% copper. I guess (assume) this is the case here as well.
Between 1914 and 16 there were some lots with K67. These were mostly SmK (AP) but not all, according the lists of dutch. I assume it is also indicating the copper content of the case and not the bullet type.
In August 1914 a new „S“ cartridge was developed for the German air force. The name was; “S” with a heavy bullet.
At the same time, they also developed a bullet with a steel core. They called it SmK (Spitz mit Kern).
It had a better performance than the heavy ball, so this cartridge was introduced.
To make a difference against the normal “S” cartridge, it became a special head stamp.
Instead an “S” a “K” from Kern, (core) in the head stamp.
By the introduction from the 67% copper case the head stamp was K67.
In 1917 they thought the red anulus only was a good identifier and the cartridge became the S67 head stamp as all other brass cased ammo.