It's not a one off trench art! My father has one of these, exactly the same as your model. He served with 10 Sqd on Halifax's at Pocklington.
The origin of these brass models is quite simple: My father informed me that enterprising individuals would make a model to cast a mould, gather shell cases, melt them down, pour the molten metal into the mould and then sell to servicemen the resulting rough cast model. The purchaser had then to rub down and polish to make a finished item. This was the case with my father's model. He borrowed files and emery paper and made a very passable model. He also has a Spitfire made from airframe white metal airframe alloys supplied by the same chap.
Apparently a fair number were made! Do not confuse these with cheap Indian sourced rubbish floating around, the originals are dimensionally quite accurate.
Sorry, I don't have photo's, but next time I'm with him I'll try to remember to take some shots.
Regards
Chris