Dear Driftless,
You have a good eye.
Vultee based their P-66 Vanguard on the same airframe as their BT-13 trainer. The original concept included three types of trainers plus a light fighter based on the same parts kit, but in the end only the BT-13 was built in significant numbers: 9,525. BT-13 Valiants remain popular with Warbird collectors.
North American tried the same thing with their AT-6 Texan trainer, but only 13 P-64 light fighters were built and only one survives to this day.
Australia was more successful their Boomerang light fighter - based on NAA’s T-6, building 250. Boomerangs were unspectacular as fighters, but earned their spurs attacking ground targets and as forward air controllers/artillery spotters.