More than one method was used to secure a swivel and tilt chair mechanism to the chair base to prevent leg base separation while being carried from one area to another. Replacement mechanism #402P will not provide that feature. The lack of that feature will not affect the stability or chair's use nor cause annoying rocking.

Fix-
Drill a hole into the solid steel 4 1/2" spindle center end and Tap threads to match bolt. Install and tighten bolt with large washer, but not so tightly as to prevent swiveling.