That is a
20mmx102mm cartrigde ( being first number refered to bullet diameter and the second to case lenght) with a TP bullet (Target Practice)
Is not from WWII, but a little more modern, it was introduced for the Pontiac M-39 20 mm revolver gun used in the Northrop F-5 Tiger II.
then was used in the famous M-61 Vulcan, the M61 20mm cannon is a proven gun, having been the US military's close-in weapon of choice dating back to the 1950s. The F-104, F-105, later models of the F-106, F-111, F-4, B-58, all used the M61, as does the Air Force's F-15 , F-16 and F-22, and the Navy's F-14 and F/A-18. The internally mounted 20mm cannon system is common to all versions of the F-15. This system combines the widely used (F-4, F-16, F-18 ) M61 cannon with 940 rounds (A through D models) or 500 rounds (E model) of ammunition.
Even the very new F-22 Raptor have an internal M-61
Also is used in some german and french guns.