The two 7 tooth boxes/columns that recently passed across my work bench the worm was "slip on" to the shaft. This surprised me since my previous box rebuilding experience was with a two tooth which is a very solid "press on" arrangement.
Examining the shaft I saw that the shaft includes a "shoulder" that is made into the shaft, just below the clip you show. Hence "gravity" makes the shaft drop to the bottom restrained by the lower bushing. The clip appears included to limit "up" movement against the worm.
Meanwhile, the worm is restrained on both upper and lower sides by the thrust bearings (same bearing used on the Model A Kingpins)
Thus the two parts are held in relation only by the 6 sided spline. A lateral/longitudinal movement is possible but limited.
Note also that the rebuild kits usually include a variety of 4-holed gaskets for use with the lower bushing. The gaskets act as "adjustment" for the tightness of the thrust bearings. Too little gasket and the bearings "bind" - too little and it introduces "free motion" to the sector. Some vendors (Berts) will sell copper/brass gasket shims for this area. Most "kits" are common brown paper which seemingly work fine.
Joe K