Yes, that's the base of of the 20mm cartrige around primer pocket made in 1942. The F.CO. abbreviation is the manufacturer's initials ( headstamp ) and may indicate the Frost & Co, Kenosha, Wis., USA.
Also, the triangular crimp marks at the primer indicate it was a reload. Recovered brass was reloaded once and used in gunnery training, not for combat.