Until this evening I was not aware that Cumont’s The mysteries of Mithras existed in a 3rd French edition, published 10 years after the English translation of the 2nd edition. But it may be found on Archive.org, and the note to the 3rd edition on p.xiii contains a couple of interesting remarks.
In response to the wish expressed by some correspondents, we have, as in the second German edition, added some concise notes which will permit the reader to verify quickly the evidence on which our assertions are based. These “texts and monuments” will be found reproduced in our editiomaior. Those which have been found or reported since 1900 have been briefly listed in the appendix, as far as they have become known to us. This small volume will serve thus to some degree as a supplement to our Mithraic Corpus. For the same reason we have introduced some new illustrations of statues or bas-reliefs, which have come to light recently. The index has been brought up to date and a good number of new names have been added to it.
The lack of footnotes to some pretty bold statements is one of the frustrations of the English edition; the French version was better equipped, but it will be interesting to see how useful the 3rd edition was from this point of view.