US Cartridge, 20mm HE-T, Mk 4 Mod 1 - 28, Mk 7
The Mk 4 and Mk 7 rounds are identical except for the dimensions of the tracer cavity, which is slightly greater in diameter and length in the Mk 4 than in the Mk 7. This somewhat reduces the tracer filling of the Mk 7.
Nothing else to see.
Collaborative Ordnance Data Repository