I think late 70's to very early 80's.
The handling of the dropout/stay intersection is not typical for earlier.
I am not sure about a C37. But I do think above a C70.
Keep in mind that with a fairly small order of "like" bikes, an importer or even a shop could spec a unique bike.

Way back in the mid 70's the shop for which I worked did just that often had custom models created including a run of 25 C37 "Artisan" frames, special color, 5/10 tubing, selected lugs, shell and crown.
These were to compete against the Italian big name bikes, they lacked the high quality graphics the Italians had at the time and a few other details such as many braze ons.
Bertin was not ready to add much to the lighter gauge tubes at the time.
Pretty rare frames, I have never seen one show up on ebay yet.