This is a 17k Yellow Gold Secret Message Holder Pendant
The Top Comes Off
Measures Approx. 1" Long and 3mm in Diameter
Weighs Approx. 3.3 grams
It Looks Like There is Hallmark But I Can't Make It Out. Tested as 17k
Has Some Wear, but Overall Good Vintage Condition
S/H is $5 within the U.S.