Thanks for that, everyone. I knew it looked familiar. Your replies led me to pictures of the Merino version, which I have seen at engine shows.
Unfortunately, it has had a throttle cable conversion (possibly never finished) with brackets brazed on right where the tag used to be on the cowling.
The only stamping I can find on the crankcase is tiny and has the familiar Department of Defense arrow above "1956".
I'm guessing that's how old it is. A national library item shows it once had a fuel tank underneath, which the pickup confirms.
It turns with a little compression, so I will tinker with it in the hope of getting it running.
Thanks again, Jarrod.