BARN FIND / Help ID this Engine

[Calahan]

Can anybody identify this engine? The Engine Number ID shows *AAxxxxxx*. There are many differences in this engine. Attached are some photos. It has a different Oil pan; the camshaft has more journals, and takes a look at the piston skirts. There’s a split on the skirt with a small spring clip on the inside of it. I am guessing a tractor motor. Thanks for looking, Calahan