I assumed that it was a Mac based on the similarities to a Mac 25 cc I got from CH about 3 years ago. The crankcase is identical to the 25 and the crankshaft is identical. I got it from a string trimmer that was several years old and don't remember the brand. The cylinder base has the mark M470 on it.
It has a bore of 1.496 and a stroke of 1.1 which is 31.68 cc.

If it isn't a Mac then I have no idea what it might be. It appears that the liner in the cylinder is nicasil, but I'm not sure.