Hi Aimee, Czechs and Slovaks became united in 1918, before then jewellery from this area were called Bohemian and sometimes not even stamped. After WWI, the jewellery were marked with 'Czecho-slovakia', 'Czechoslovakia' or 'Made in Czechoslovakia'.
The c clasp makes think late 1910s/early 1920s maybe? Is the front of the brooch enamel?
Very sweet