Just flipping through my three copies of Alan C King's books and not much jumps out that fits your description other than the Copar you mention. One I see is the Laurel from Denver, CO; it is dated as 1954 in the book. Looks like 16" rear wheels and a dual-wheel tricycle front. Another is the Bantam from the same time period. The pictures show and list lots of implements that were available.
In addition there are some silly looking little things that would hardly even be considered a lawn mower these days - examples are the Strunk Co. Chipmunk and the Hiller Engineering Corp. Yard Hand. They both look like the "big" tires have no more than 6" rims.