75 MM Inquiry

My latest garage sale find is a projectile of a 75MM canon. Does anyone have any information, I have done an internet search and it is leaving me blank. It is 8" OAL, 2.940" diameter, the base has a 4" deep hole drilled into it that is 1 1/2" diameter with threads near the base. The markings on the base are “1942 GMC 75MM M61 S2 ???-22586”. I am assuming US manufacture, am I correct? The cavity in the bottom is too large and threaded to be used for a tracer element, in my opinion. Can anyone give any information as to the loading and use. Thank you as always.

US 75mm APHE the large cavity would have been filled with HE and a base fuze would have been in place on the bottom or back end

What you have found is what’s left of a 75mm M61 A.P.C. shell.
As you can see there are cuts at the top were normaly the cap fits.
On this penatrating cap there is a ballistic cap.
The drivingband is gone.
The hole in the base could be closed with a steel plug with a tracer , empty shell .
But more likely there was a Base Detonating fuze. B.D.M66A1 , shell filled with Explosie D.

This is the "75mm Armor Piercing, Capped- Tracer projectile, M61."
You have the basic part of the projectile, but it is missing the hardened steel cap that gave it the armor piercing quality, and also a sheet metal ballistic cap or windshield that provided better ballistic performance than the blunt cap.
The large threaded opening was for a base fuze which also had a tracer element on the back.

Here is the official description of the projectile from the Army Technical Manual for the 75mm APC-T projectile which was identical except for the type of crimp for the windshield:

"The projectile consists of three parts: a steel body which contains a small charge of explosive D, a steel armor piercing cap, and a 360