well, this most likely isn't port.
Port by definition must be min 18% ABV according to the IDVP.
They'll give you that white label called a Selo to throw across the top.
This is 14.
Light sweet wine regardless of age would be very similar to the wine you find that comes in the jugs, ie white zinfindel. Very good chance that this one is maderized, and if you were to drink it, it'd probably be thin, sweet and watery.
btw the seller is actually caves do restelo who is a bulk exporter/distributor of wine in portugal. They probably got someone's over run and bottled and shipped it.
the age of teh bottle looks to be around something from the 70s
unfortunately, I'd say the bottle is probably worth nothing and i'd personally keep it around to tell stories.