I just gave the manual in the link a quick perusal, and didn't see reference to a spacer ring. Here's some info from another website:
Spacer ring, for use with Leitz 12X16" easel and Leitz 1c enlarger. I have had several of these easels over the years, but none came with the spacer ring. The Leitz 1c enlarger is designed to autofocus on the baseboard or on a 25mm high easel. The Leitz 12X16" easel is thicker than 25mm, so a spacer is required. The spacer goes where you see it for use with the Leitz 12X16" easel.
See attached photo, the ring is blue, and sits above the pin.