Thanks to Brian for sending in photos of this engine (owned by Art Gaier), the following is his description of it:

It is three double-acting cylinders, 120 degree crank-offsets, with
through piston-rods so no cross-head guides are needed.

The "valve" chest appears to be that round tube low in the first
picture (658a), and each crank-throw has a valve rod direct from
the throw to the valve, so I assume this is some sort of rotary
valve. There is a definite "supply" port and lever throttle, on the
right in 658a, and head-on in 658d. The lever appears to be
connected to something having to do near the larger chain-wheel
sprocket. What is NOT apparent is the "exhaust" port, unless it is
provided at each slot in the valve-tube where the valve rods
connect/enter.

658a. Assuming that this is the front view, approx. 9" long:




658b. Left side




658c. Back, some mounting holes are visible in this photo:




658d. Right side





658e. Top





658f. Bottom




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