It is really hard to make out the stamping but looks like 990! It can't be right as 990 refers to 99% pure Gold by U.K. standards and the pendant does not look like Gold. Can you take and post a better picture of the stampings (i.e. a close-up and show them more direct or level as they look tilted)?
I really can't tell you and would suggest taking it to a Jeweler to have checked! They look like "9's but that is not a "0" (zero) and I don't believe they are "6's". I can't make out what is under them!
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.
The circular mark does indeed appear to be a copyright mark - no idea on the what the two numerals signify - the metal doesn't appear
to be silver to me - from those pictures anyway .