Rolex 1980s Model - Doubt

[andrepd42]

Good afternoon, everyone,
I have read the post regarding value/prices and selling watches, but this is not one of those!!! Its a legit doubt!
Im 20 years old, and my grandfather just died, and my grandmother found a watch of his that she was not able to tell me if it is a replica or real watch (my grandfather had both...), and gave it to me either way... She believes its from 1980s...

I only came to ask you guys, because I tried searching online for this model, and didn't come with any exact hit... So I just wanted to know... Was there ever made this model? or is those horrible fakes, that don't even have a real model alike, hahaha)

It is a Oyster Perpetual, stainless steel, automatic, moving about 5-9 times per second (can't tell exactly), acrylic or alike glass...
Has a steel band with 7835 and 17 writing in one side, close to the case, and 251 on the other side...
If I put my ears to the back of the watch I can hear movements, though not loudly...

From what I know rolex uses sapphire crystal, so I got even more doubtful about the originality of this watch, but I do not know a lot (to be truthfull anything, about Rolex watches... My best watch is just a Tissot Automatic...)