Dating by correspondence 1950s
Prior to the early 1950s, typical necklace clasp styles used by Coro and Trifari were spring ring, push (also known as box), or fold-over.
Since Hollycraft was not a huge manufacturer like Coro or Trifari, and had a more defined style, this information is not of great use in identifying general styles of the decade.
Researching Costume Jewelry at Way Jewels at Glory Antiques at Mary Ann Docktor-Smith is a vintage jewelry collector, historian and dealer.
She also creates artisan jewelry using vintage and natural stone beads, polymer clay and cork.
The other types of clasps continued to be used, and are still used today.
However, if you were dating a necklace that has an original hook-and-extender clasp, you could begin your research with the assumption that the piece was likely made no earlier than the 1950s.
A second primary-source dating tool is the use of books about fairly-large and prolific costume jewelry manufacturing companies, where original sources have been used by the author.