Funny... My first guess was that those areas are the result of exposure to vinegar or some other form of acetic acid. Older glass, such as found in antique glass bottleware, can be prone to developing matte-like splotches. That said, I would not necessarily expect to see such loss of glass sheen in your particular bottle, as it strikes me as a little too 'young' to show evidence of that phenomenon.
However, you now confirm that the areas lie beneath the surface of the glass. Strange indeed!
I also note that there are obvious diagional scratch marks to the right of the lower portion of the label.
All of these factors raise a smidgen of doubt in my mind concerning the bottle's authenticity. I'm not
saying that it has been subject to tampering, but am merely stating a slight degree of concern.
Therefore, it's worth checking other parameters of the bottle in question:
1. Is everything else (proper unmarked cork stopper with tan-coloured wooden top as well as silvery foil capsule with subtle horizontal circumferential striations and no punt - i.e. indentation - on the bottom of the bottle) in order?
2. Fill level. I would be a little wary if the fill level in a bottle of this age reaches much higher than mid-neck level, as that could possibly indicate refilling.
Other than that, the very best way to discern authenticity might be to open the whisky and have a go at the contents.
Best of luck.