Jim, note the vertical 2x4 behind each side of the "log cabin'd"
wall boards. In concrete form work, we call these "strongbacks"
and they make for very solid corners that will not blow out under
the extreme hydraulic pressure of many feet of overhead concrete
while being vibrated to settle out rock pockets.
I assume this shed was hand/shovel loaded, so the stress of a big
dump would not occur, but I supposed that coal gave enough outward
push to make this construction attractive.
"Duty above all else except Honor"