Sadly there was no picture but the device was described thusly:
2) a air compresser from a small block ford. 4 cylinders fire 4 pump air. Propane ran. 125 cfm constant $225
Hmm fascinating. Not sure why I haven't run into this idea before (or if I did why I forgot it!!). Bears some thinking about maybe...
Some quick interwebs searching and we see that its not a unique idea by any stretch.
From a brigs:
Possibly even more bizarre - this guy is using two cylinders to drive the engine and the other two as compressor pumps.