As a general rule of thumb, a C Clasp, without the locking part, indicates that the pin or brooch is old. There are exceptions to every rule, of course and this one is no exception, so other details such as the setting, type of stones, etc have to be taken into consideration to determine the age of the piece. Most modern C clasps can be indentified because the metal looks newer and is less well made. Old C clasps look to be more of a permanent part of the pin, rather than an addition. The cameo pin to the right shows a very good example of an old C Clasp.
Photocredit : jewelryexpert.com
A C Clasp is named because the part of the clasp that holds the pin in place is formed in the shape of a C. The earliest ones are just a rounded wire. Some other early styles include a more solid piece with a flattened top. As you can see from the photos below, there is not much to hold the pin in place.
Most brooches used C Clasps up until about 1900. Around this time, the locking C clasp was invented which used a locking mechanism to keep the pin from falling away from the C. Most brooches made today use some version of the locking C Clasp.
Here are several examples of very old style C Clasps:
I have brooches with old style C clasps in all of my web stores. (locations at the top right of my blog). Here is a very pretty Victorian Ribbon Bar Pin with a C Clasp from my Carolina Collections Vintage Jewelry Store on Ebay. It's available for only $26.50.