Swivel Base.
Designed to allow the indexing head or "Universal Adaptor" (below) to be mounted at an angle on the table and work machined at an angle or taper.

Universal Adaptor. This device could be fitted either directly to the milling  machine table (so that the swivel table was aligned at 90° to the headstock) or mounted on the swivelling base above - so allowing work to be machined at any angle. The 10-inch long T-slotted table could accept any of the usual table-mount accessories including indexing heads and tailstocks.