ID of old ring

[Canucky]

Hey guys,

I have a ring that was passed down from my grandad and I would like to find out some info about it if possible. I actually assumed in might be costume jewelry but now I believe at the very least that it is authentic. It would be kindof nice to know a little about it for sentimental reasons. Really appreciate any information.
The marks are:
J.H.18CT PLAT and
PA57.

http://i245.photobucket.com/albums/gg63 ... ing002.jpg
http://i245.photobucket.com/albums/gg63 ... ing003.jpg
http://i245.photobucket.com/albums/gg63 ... ing005.jpg
http://i245.photobucket.com/albums/gg63 ... ngy002.jpg

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