Was it in loose soil as obviously it was just buried as no rust on the iron and the blade even has a shine to it on the edge.
Had it been in the ground for a long time all you would have found is the aluminum handle.
when i found it, it was in a park, the soil was dense packed, sounded like a quarter... the blade was all rusty and the t handle would not open.. I put penetrating oil on the joint of the t handle and i took a small wire wheel to the blade to clean it up and coat it with oil. i believe the park has a date some where in the 1930's and i dont know if fill had been brought in or not, maybe i can do some tracing to find out thru the city.. after i got it all cleaned up and working, i went on line and printed some of its history out of the swallow airplane company ( they made bi planes) and then gave the screwdriver and printed history to my son in law who collects old tools ... to say the least he was most over joyed at having it..
the handle appeared to be alum.
The soil was very nice to that tool and must have been really dry. The hole in the handle held a spring loaded tension button that held the handle in place when closed. The maker is KP Enterprises here in Denver, CO. Mine says pat pend. Here's it's sibling;