These were cheap revolvers that originated in France, intended for bicyclists as a defense against against dogs attacks. They fired a curious centerfire cartridge, the 5.5 Velo Dog, that was similar in size to the .22LR, but much longer- and it was a centerfire cartridge:
The Velo Dogs were loaded with a jacketed bullet, as opposed to the heeled lead bullets used in .22 rimfire cartridges. The bullet diameter was the same as the standard .22, but the case diameter was larger than the bore, like most modern cartridges.
Askins managed to buy up or otherwise acquire several thousand rounds of of these obsolete cartridges. He pulled the bullets on several hundred, and shortened them on a lathe to .22LR length. He then found a supplier of lead bullets in the correct size, a set of reloading dies, and had one of his Colt Woodsman pistols modified to take the new cartridge, which he dubbed the .221 Askins. His load was a cast lead semi-wadcutter over about 1.5gr of Bullseye or DuPont #5, depending which version of the story you read, and with it he won the All-Around pistol championship in 1937. Following that match, the NRA amended the "any centerfire" pistol rules to read "any centerfire (.32 caliber and above.)"