There was no change in lume material, Omega used tritium all the way from 1963 to 1997 but some models do lack the Ts it is true. The Bond SMP is the most obvious, launched 1993-4 with tritium, no Ts in sight. The earlier Pre-Bond was the same. It does seem strange that some models have Ts and others don't but it is a thing, particularly from the 1980s onwards.