Syncromesh unit assembly rings
18G222 - Second Speed
18G223 - Third Speed and Top
This tool is specially designed to retain the balls and springs in the synchromesh hub while it is being pushed into the sliding sleeve.

The pictures show two different versions of the same tools.
Picture on left is from the MG Series MGA Workshop Manual.
Picture on right is from the MG Series MGA Twin Cam Workshop Manual.