EUREKA!
Hello, I needed to think a little about it but I came out with a possible solution: that adapter was meant to replace tha main telescopic handle of a german Vorwerk vk 120, and allow to connect the electric portable handle that was provided with the original "Polster boy" whan it had to be used not connected to the vacuum cleaner in the upholstery shampooing mode. This would have allowed Handling the unit with it, I don't know for which purpose. In Italy Vorwerk vk 12o was designed with a three-piece handle: the lower one was green plastic and had lower cord hook and, power switch, and cord attached, mid steel handle and plastic handle grip with upper cord hook. When using the cleaner with attachments you would remove upper handle so you didn't need anything else. A side handle with shoulder strap could be easily fitted. On German model the handle came in one piece, telescopic, with power switch at figertip on the handle grip and power cord coming out of the handle. None of the instruction manuals in my possess show this adapter!
Greetings, Tommy