Hi
@CChin, welcome to the forum.
I can tell you your grandfather's watch is 100% authentic. - (subject to seeing movement).
You will find the model number on the inside of the case back. Let your watchmaker remove the case back as you could cause damage.
It's likely a ref no.168.005 or it could be a 14902 if one of the last models. At a guess I would say it dates to around 1967 give or take a year or so. If you post the movement number we can tell you the year.
The winding crown is genuine Omega but it's not original to your watch. It was probably replaced many years ago as part of a service. Lots have incorrect crowns so no big deal really. Originals are no longer available but with patience they can be found on eBay.
In regard to the gold medallion these do work loose and fall out occasionally. This will prove more difficult to find so worth giving your parents house a thorough search.
The bracelet is called a 'beads of rice' and likely came with the watch when new. A leather strap would have been the other option.
The dial, hands, case and crystal all look original and in very nice condition.
It's a nice watch and worth investing some time and money, maybe get it serviced to bring back up to scratch. Ask here for watchmaker recommendations in your area.
Good luck.
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