Absolutely 21.15mm. I was actually scrolling down hoping that nobody answered yet...I'm never the first one who knows the right answer, dammit!
Anyway, Schwinn, in the 80's, used Tange tubing for their fork steerer tubes that was of the same outer diameter, but the inner diameter was smaller than one inch because the wall was thicker on the tubing. I used to know why, but I'm not as into vintage Schwinns as I used to be.