Obviously there is an overlap of different components used and you must bear in mind that sometimes a broken clasp will be replaced with a more modern version. Also this type of clasp is still sold today and may be used by some jewellery makers.
So you will need to date the whole necklace or bracelet generally.
Around about the late eighties to early nineties, the lobster style or claw clasp came into use.
Now used extensively on new jewellery manufactured.
Other clasps will be discussed at a later date on this blog