CR FOLK:
I reckon y'all are mainly concerned with when recessed brakes came to be made by Campagnolo in particular. But as an additional data point the Zeus 102 catalog (with a preface dated 1973, but a 1972 print date on the last page) already shows the Zeus-2000 brake set with recessed bolts. It also shows the Zeus-2000 6-speed aluminum-cog freewheel.
Oddly, a picture of the top-model bike in the same catalog appears to show the front brake recessed but the rear brake nutted.
As to when recessed brakes became available on production bikes: my circa '76 Zeus Supreme (a standard production bike) has 'old' features like: no brake cable brazeons, no shift bosses, no bottle bosses, top-of-bracket shift cable guides, yet also has two 'new' features: six-speed spacing and recessed brakes. My '74 Zeus (still has the 'ribbon' headbadge that recalls the earlier Alfa logo) has the exact same specs.
On the other hand my circa '81 Colnago Super has 'new' brake-cable & shift boss brazeons, and 'old' 120 spacing and nutted brakes.
Go Figure.
Brad Stockwell
clamp-ons & braze-ons honored equally
Palo Alto