1976 sounds about right. They're in the 1975 catalogue:
and they feature in a number of print ads from 1976:
but they're gone by 1978. Velobase has a little information:
http://www.velobase.com/ViewComponent.a ... 2&Enum=115
The bolt circle diameter is about 95mm, with spare rings available down to 30t. I don't know of any compatible rings: the more common 3-arm cranks used 116mm bcd.
Calmes dans le demi-jour
Que les branches hautes font,
Pénétrons bien notre amour
De ce silence profond.