For the 105 years they've been manufactured here in North America, our manufacturers have certainly played fast and loose with the terminology. They call it .410 gauge as often as they call it .410 bore. Remington ad circa 1930 --
.410 Gauge Ad circa 1930.jpeg
In the early years the most often included 12 m/m and 36 gauge on their packaging --
2-inch .410-bore.jpg
FIELD .410-bore 2 1-8 inch.jpg
FIELD .410 12mm 36ga.jpg
Here at least Winchester just used .410 and (12 m/m) --
.410 (12 mm) Paper Shot Shells.jpg
But here they used 410 GA --
410 GA. SUPER SPEED.jpg