Where 2 mount ball turner ???

[PixMan]

If you do indeed take it apart, you'll find out quickly when you flip it over and see if there's enough mass to bolt the base of the swivel directly or a need for an extended plate. Many cross slide tops are "beefed up" thickness around where the compound slide's T-slots are milled, but hollowed out beyond that as it's more a cover and connector/spacer for the dovetails than a support for additional mechanisms.