Just had a look at a cowl pic of a 40 wagon I sold a few years ago. The tag in question is a generic to 40s and I believe is a patent plate located on the center/left/upper firewall attached with 4 rivets. If you remove the voltage regulator you should see a series of numbers stamped into the sheet metal. My 40 was #10085. Its just a point of interest. I think 40 convertables have a body/cowl number stamped there as well.
If I remember correctly Kube mentioned in an earlier post that there was also a tag affixed to the cowl on the drivers side upper cowl not sure what its function was. I'm sure he will chime in as he was interested in others who might have the additional tag attched... Bill in Nova Scotia.