1977 Datejust Authentication Help

[bleep]

I purchased this watch on eBay not too long ago from a seller with a good history.

I posted a picture of the watch a couple of days ago and I have had a few people show concerns that the magnification was off.

I took it to a few jewelry stores and they right off the bat said it was authentic (without looking at the internals). I would be much obliged if you all could take a look and help me set my mind at ease! One of the authorized Rolex dealers told me vintage Rolex watches may not have the same magnification because it is an acrylic glass -- not sapphire. I have not heard of that and thought they were all 2.5x.

Here are a few more pics http://imgur.com/a/YQv5F

Thank you!