I have heard many samples of all 3 versions.... I own an early and late. Not sure I would say there is a real noticeable difference. Maybe the later version is a tad bit brighter? But that could be 5 years younger caps....?
I do like my early version slightly better.... Serial 10400 something better than the very 18000 something... ( just don't remember the exact numbers ).
The difference are in the transistors, some caps, some shielding, 250k vs 500k volume pot.....a few other minor differences.
The face plate is clear coated on the early one and bare anodized on the late.... I tend to like the clear coated better....
The middle version is a mix....
But honestly I would not get too hung up on that, as they are all great sounding once refurbed and modified....
JohnK