That is an excellent set. I believe the markings are period. Though manufactured in Germany, this is an Austrian issue and the markings are typical of an Austrian unit. The F.K.25 stands for Feldkanonen Regiment Nr. 25. The 140 is the man's matrical number. I'm not sure about the "J", but it could be an "I". The 1/4 probably indicates the 1st Zug (2 guns) of the 4th battery.
Most of these pouches were made in Berlin, by various manufacturers. I can see "Berlin" below the manufacturer's name. One of my two examples also has a Berlin maker, but a different one than this. I think yours was made by Paul Loewenberg Sohne, which is the most commonly seen mark. You can sometimes see Austrian acceptance ink mark on the mask out to the side of the eye pieces, or stamped into or ink stamped on the filter canister.
These second pattern pouches (two compartments) sometimes still have the spare filter tin, as does yours, but the more difficult tin to find is the one for the mask that came in the first model (three compartment) Segetuchtasche.
Chip