I just came across this website with a large number of WWI era photo postcards of military vehicles and equipment that can be viewed online and thought I'd pass it on. I've just looked at the first page and have noted a number of rarely-seen armoured vehicles, artillery tractors, & etc.
When you see an image of interest, just click on it for an enlargement. You can then enlarge some images further by clicking on the arrow symbol down in the lower right corner of the image.
Disclaimer: I have no connection to this website and have never used it to buy or sell items.
Enjoy, and share your interesting "finds" with the forum!