Those external data plates under the stabilizer where added to all US registered aircraft in the late 70's when the FAA made it a requirement to have manufacturer, model, and serial number placed on the exterior in that location. I think this was to make it easier to attach true aircraft ID for the "drug wars" at the time (smugglers where changing "N" numbers all the time with water based paints to fool the DEA).
On my 1940 T'cart I had in the 70's and 80's and the 1946 14-13 I had in 1986 I actually painted that data on 'cause I didn't want to glue a metal ID plate on my fabric.
The factory data plates are on the interior of the cabin, the location varies.