Not a paint inspector stamp but for another operation/test. History of the small or even larger "base ball diamond" style goes back to at least the early 60's at Dearborn. You'll find the smaller ones connected to electrical operations allot of time. Paint inspector stamps (execept for one older inspector that was moved but filled in every so often when needed) changed to paper sticker style in late 66 at Dearborn. Of the base ball diamond style in the early years you can find Paint OK, Final OK, Trim OK, and Tested as you found. We see a return to ink/paint stamped inspector stamps in late 60's into the early 70's
One of our members had found a two line "Tested" and "Ford" style stamp also on a 68 Dearborn example in the past. Not sure why there were two different designs but maybe one was a more recent replacement and changed during that process
Here are some other examples similar to that one you found and posted
Thanks for sharing