HD is high definition, chiefly the polarising material sandwiched between the glass, very similar to what B+W Kasemann POLs use: it is colourless, extremely thin and very, very clear; this benefits digital shooters, not so big a deal for analogue. Not all polarising materials are or were ever created equal. The Chinese made polarisers are appalling, masquerading as "quality optics". Buy one and pull it apart and compare it to a B+W (though it's not recommended you pull one of those beauties apart! )

I've had perhaps 2 or 3 POLs with damaged threads over the years; they are routinely thrown out; damaged threads on the lens side will damage the lens threads; frontal filter thread damage is by and large inconsequential, perhaps not allowing a hood to be attached (unless a clasp type). A dent on the front of a filter proves it has saved your lens from the same thing!