As Griffo has mentioned, the top of CTA back in the 70's was FL400. The Lear 24 BSJ frequently operated EN-SY at FL410 both Northbound and Southbound. The pilots always objected to being told to squawk standby and call FS on leaving FL400 on climb!! However we always kept ident on them regardless, for they were certainly part of the traffic mix outbound and inbound to ML/EN. Those were days when transponders had just been introduced and ident was kept on each aircraft by "shrimp boats".
Between AD-ML-CB-SY and ML-Tasmania, the CTA base was 10,000ft.