hello Graham, and thanks for the interest.
the name derives from the first 100 year anniversary model, the Centenaire, which was released in 1956. the release of this model, designated the 61, is a total mystery to me. they made one more centenaire and called it the 71. i haven't done any research into the naming, although other eterna devotees have had good success in contacting the company with questions, i just haven't done it yet. the centenaires seem to have been the top of eternas line when they were released and this model features the cam micro regulation (pic not mine):
this was a new feature for eternas as the previous micro regulation was the whiplash seem on teh chronometers (pic mine this time):