I've also been looking into this thread for quite some time, and I'm waiting for one thing and one thing only- the movement picture.
As for now, I'd lean towards considering (not to be confused with "I'm convinced") that this watch is original. Apollonaught has posted a watch which appears to be identical with OP's- the ref. 3417.
As to the bracelet being a probe of whether this watch is genuine- I wouldn't suggest myself with that. I've seen people replacing original bracelets with a mesh, just because that was their preference. It all depends on the attitude of the owner- if the OP's grandfather simply wanted to have his watch on a bracelet, no matter how it looks, then I can see no reason for which he shouldn't have done so. People tend to have different definitions of what "aesthetic" means. I've seen people placing a Speidel bracelet on watches of reputable brands. As much as I disagree with their choice, I can understand it.
The last thing that convinces me, is that it was identified as original by a watchmaker, who was dealing with Patek for apparently quite some time- and probably had the knowledge necessary to distinguish the real thing from rubbish.