The early 90 swivel has a railco top swivel bush and large top pin. The early discovery and range rover have a smaller top pin and a taper bearing which would make interchanging just the chrome ball a no go. How ever you could swop a complete stub axle and chrome ball from a discovery to the 90 with ease. Just remember that an early disco front half shaft is in 2 pieces and the 90 half shaft is a single complete unit.
charlie.