I'm quite smitten with Peter Pan collars these days---- I'm not sure that it was fashionable in the 70s, but I do remember wearing clothes with this style of collar.
The project above is based on Colette's free downloadable Sorbetto Pattern. The added Peter Pan collar is outlined in Sew News 3-for-All article in the April/May 2012 edition. In fact, there are a few different collar options described in the article--- they all look fun!
More on Peter Pan Collars---
I have never worn a detachable collar, but I'm intrigued by these: