I'm just very curious, I can't find a pure RPM mower mentioned anywhere and it seems they were made for several years before LB bought them in 1952
1946: Joel G. (Jack) Doyle built the first rotary lawnmower for the Rotary Power Mower Company of Kansas City, Missouri. Doyle accumulated large orders for these mowers from Sears Roebuck, Gambles, Spiegel, and other businesses. Evidently, the demand for this machine was thriving.
1952: OMMC purchased Rotary Power Mower Company, located in Lamar, Missouri. OMMC then changed the brand of that mower to one that they already owned Lawn-Boy. The RPM facility was converted to production line manufacturing in order to meet the large orders demanded by retailers such as Sears Roebuck and Spiegel. This style of production was the first of its kind for walk power mowers.
1956: The corporate name changed to Outboard Marine Corporation. Lawn-Boy took the lead in the production and marketing of walk power mowers through independent retailers.
That's the one! Mine had red wheels, and since it was sitting in a barn for probably about 40 years before I got it, it wasn't really in that bad of shape. Only real problem was the muffler was completely rusted away, and I never did find one that fit, so it was a bit loud. When mowing it wasn't bad since the grass, etc. muffled it a bit, but on pavement, it had quite the crackle to it.