While the usability & features of the 5-20HD hasn't changed much there have been at least 2 production revisions I'm aware of.
The first batch had the parallax knob marked to 35Y on the bottom end and had 60 splines on the windage & elevation knobs causing minor alignment issues between the hash marks on the turrets compared to the marks on the scope body. With 100 marks on the knob but only 60 splines available to align the knobs the marks would usually not line up when you moved the knob to zero the scope.
The second batch had the parallax knob marked to 50Y on the bottom end and 100 splines on the knobs to fix the turret alignment issues.
It wasn't just the marking on the parallax knob that was different, the first version scopes could be in focus and parallax free at 20X closer than the second run versions. The 35Y and 50Y marks on the parallax knobs were pretty close to what the scopes were really capable of at 20X.
Can't say if there is a 3rd or 4th revision as I haven't owned a 5-20HD in a while, but I had 9 of the illuminated 5-20HD at one time and those were the distinct differences between the first run and second run models.