If it is WW II or earlier then it likely has a Radium painted dial. IIRC a 26,000 year half life. Tritium was used post WW II most likely starting in the early 1950's.
Hugh, retired Nuclear Safety guy who had the dubious experience of mapping the Radium contamination of the Elgin clock factories in northern Illinois. This was one of the places that the Radium Girls worked painting clock, watch and instrument faces..