Brass vs Zinc dash plates

[TimG102]

I am not sure if this has been covered some where else, but can anyone help explain which type of dash plates are correct for different manufactures, models and years? I am working on an M3 Autocar (1441) and looking to put new plates on the dash. I would have to assume changing to zinc was probably to conserve brass for the war effort. I believe I have seen a lot of tracks with a mix of brass and zinc plates. Any insight would be appreciated.

Tim

[col.halftrack]

 Tim,
 Not sure on this but after seeing several tracks I feel that the manufacture's used brass (main data plates) from start to finish. The steel / zinc ones I have seen are all ordnance re-build plates. as far as the smaller plates ??
 Great start to a good thread though. we will see what others will say.