Ken
The only "top slide sole plate" reference by Bradley that I know of is in his ML7 lathe manual, and it is simply a raising plate to lift the complete top slide together with it's base.
As you know, normally the rotation of the topslide is limited in each direction, the sole plate enables it to rotate completely, without the topslide handle hitting the cross-slide fittings.
Don't think it will help you, as it makes the topslide higher.