US Projectile, 40mm AP, M81A1
This projectile is an Army design manufactured for the Navy to Naval specifications.
The AP round has a plug in the tracer cavity; the AP-T has a red tracer.
The M81 cartridge differs from the M81A1 in that its windshield is attached to the projectile body by an adapter.
Nothing else to see.
Collaborative Ordnance Data Repository