Looks like a letter "C" inside a circle which is a symbol for "copyright design" here in the U.S. I have marked some of my jewelry designs with that mark. The 66 after it could be the year it was copyrighted, as in 1966. If the other mark isn't "sterling" or another metal purity mark it is probably the manufacturer's name or mark and the piece is costume jewelry which does often have the "C" inside a circle copyright mark.