by Record-changer »
Thu Mar 03, 2016 7:19 am
I know what it is because I have one. I also have the Webster-Chicago 356 it fits.
It goes on a Webster-Chicago 356 shelf-type record changer to extend the record shelf to play 7" records. Here is a picture of the changer with the 7" extension attached, You turn the record shelf to the 10" position and then clip the extension on to extend the shelf to hold up a 7" record.

The little lever sets it for the difference between the thickness of a 7" 33 and a 7" 45. As in the photo, spiders are needed to make the center holes of the records fit the spindle. The rest post must also be turned a quarter turn to set the changer for 7" setdown.
There should be a spring on it to clip it to the record shelf. I don't see it in your photo. It hooked into the smallest holes visible.
This was the first 3-speed record changer ever manufactured. It was a quick redesign and kludge of the 2-speed model 256 to get something on the market after RCA introduced the 45 rpm record.
They also sold a conversion kit to upgrade the 2-speed 256 to a 356, containing the 7" clip, the modified rest post, a replacement arm raising disk, a spring to expand the 33 motor shaft step to 45, and a lug to store the spring on for 33 use. With the upgraded 256, you had to take the turntable off to change between 33 and 45 operation. The 356 has a 3-position speed control the 256 does not have.
The following year, Webster-Chicago released its very successful 100 series: (see image below)
