Hey Geo,
I have not done a lot of research on these yet, but it is my beliefe that they were produced between WWI and WWII with some production extending past WWII. Right in front of the strecher bearers that are between the nurses, you will see an unusual motorized gun platform. This is a toy version of the one off "belly flopper" developed in the 30s, it had a driver and a gunner, and was tested by the US Army, but found to be lacking. Some of the toys are lead, and some are cast iron (the lead ones are more fun to chew on).
Best wishes
Gus