Anyone's guess really. For at least a decade and a half I've heard them described as trenched minute index dials, referring to the recess in which the chapter ring is placed. But, I note recently the nomenclature of railway dials has been used. Although, railway dials were, again more than a decade ago, almost exclusively used to refer to the twin track dials in Omega Centenary and other models which sported the 'railway track' index.
I know in one of his publications, Saga, Marco Richon described these type of dials as " fosse a l'index" which, according to my increasingly corroded understanding of French translates roughly as a pit with indices, hence trench I suppose.
Cheers
Desmond
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