Got these pictures of the vintage Allstate "Tag-a-long" one-wheel trailer (sold through Sears back in the day). Found hitched to a Morris Minor at the British Car Festival in Palatine, Illinois on September 8, 2019.

These were sold through Sears at least from the late 1940s through the 1960s. This is one of the early style with relatively small metal load box (nicely restored). 19-ga steel box, shipping weight 175-lb, assembly required. Apparently the external parts were shipped inside of the load box for most compact shipping. Later models had a wood box on the same basic frame, and a couple more later models somewhat different, likely a bit larger, with two quarter elliptic leaf springs holding the wheel axle aft of the pivot joint. I used to see these fairly common in the 1950s, usually clamped to the chrome bumper of a larger American car, and sometimes the larger model trailers.

You might find a more extensive web page for these trailers at www.singlewheel.com