This is a M252 cartridge used to release a cargo parachute with a 20 sec. delay.
Thank you Fede.
Just took some pictures of this earlier XM252 variant made by Picatinny in 1968.
Details from the technical manual:
Great info Fede thanks !
here are two others
The upper is lot PA-290-3 & is dated 3-65
The lower is lot FA-307-1 and is dated 4-66
Funny how the one I obtained is stamped in black ink, and the ones Fede and Pete show are stamped into the metal.