need help identifying prewar German wagon

the couplers resemble Carette circa 1902 with the large flat loop but the hook is longer. Basset-Lowke adopted these also. The only other flat loop coupler I see pictured in Gustav Reder's book is 1894-1898 Marklin. The trucks look more like the Basset-Lowke.

Good one.  You've got us stumped, and it's not for lack of trying.  In addition to Carette, both Bing and Bub had a similar hook coupler - very early.  I'm agreeing with Jim's early, turn of the century dating - the couplers, spoked wheels, and delightfully primitive tank all point to that.

Try looking here:  click around on the links and you'll find some frames and couplers very similar... but unless I missed something, nothing matching exactly.  If for example you found an O Gauge tank that was made the same way, that would get you in the right direction even if you couldn't find this exact car. But I'm not seeing anything like this; it's quite unusual.

Here's what I noticed. The tank is mounted off-center on the frame, and the steps are on one end; the whole frame is asymetric, with the wheels mounted under the tank, not centered under the whole frame.  So I'm thinking there was something else up on the frame at the end of the tank, and my guess would be a brakeman's guardhouse or "gueritte".  If that were the case, that would point to continental European (French or German) rather than British.

whoever made it, nice tanker.