The mechanic dropped his Shorpy logo in the oil pail.

Two cylinder heads with two head gaskets, three cylinders each.

I don't remember working on any motor like that.

There is a valve spring compressor on the last valve by the firewall. The pistons are probably from a different engine. It's not uncommon to have more than one job going at the same time. You got to wait for parts sometimes.

I see no indication yet that this was a valve or ring job.
All that has been removed is the head. Pistons and valves are in place as are parts that would need removal to gain sufficient access to remove pistons or valves.

This may just be a decarboning operation or they may have had leakage of exhaust into the coolant.

I agree with Dave that it looks like a ring job. Note the left-foreground ring atop its waxed paper wrapping, on top of the stack of what looks like four new ring boxes.

There are four used pistons (with connecting rods) on the workbench, but not four piston-less cylinders in the open engine.

So to what engine do these four piston assemblies belong?

And materials and designs have improved such that engine overhauls and valve jobs are pretty much a thing of the past.

[I see a ring job underway. - Dave]

Nice placement of the "SHORPY" logo

Just looking at the pic, how do we tell whether the engine's a Diesel?

Valve placement is the giveaway.

Autocar is one of the oldest American vehicle manufacturers, tracing its origins to 1897. The brand is still in business today. The company has a long list of significant achievements, including building the first commercially available motor truck in the US in 1899.

In 1943 if someone told you they'd just performed a "head job", they'd likely feel no need to blush. (Wink wink). Associated was still relatively new when this pic was taken

Horton had started out in 1930 in North Carolina.