puzzle ring with blue enamel

A sweet 18-carat yellow gold ring consisting of two thin ring bands, each decorated with a line of blue enamel, one in royal blue and the other one with baby blue enamel. France, circa 1970.

weight: 3.75 grams
size: 16.75 mm / 6 ¼ USA
width of the two bands together: 3.53 mm