German term (edited): Treibschieberz?hler

The 'reciprocating-piston' part is a guess (at least the 'reciprocating-'part), but I'm fairly confident of the 'volumetric flowmeter' part. From the various google hits for Triebschieber and Triebschieberzähler, it appears that a Treibschieber is a type of positive-displacement mechanism for driving a flowmeter, and there are basically two types of non-rotary positive-displacement flowmeters: oscillating-piston and reciprocating-piston. 'Schieber' suggest a slide valve, which makes me think this term refers to the reciprocating-piston type (which uses a piston and two reciprocating slide valves).

For detailed descriptions of various types of flow meters, see the first reference.

For a bit of German text about flowmeters, see the second reference. A quote from this reference:

- Volumendurchflussmesser
hierzu zählen Ringkolbenzähler, Ovalradzähler, Taumelscheibenzähler, Treibschieberzähler, Zahnradzähler, Schraubenspindelzähler.
Zähler dieser Bauart werden bevorzugt bei viskosen und schmierfähigen
Medien (Öle, Brennstoffe) eingesetzt.