A "V" series would have been made in 1978 through 1980.
68XXX was made in 1967. Numbers started that year at 61000 and ended at 73799.
This is usually considered to be an "early" Python these days.
The 6" is considered to be the "classic" Python, but 1960's Pythons are considered to be better quality.
I'd call this a toss-up with the edge going to the 4" simply for it's earlier production.
As a shooter, it depends on whether you prefer the 6" or 4" barrel.
With Colt having stopped production of the Python, apparently forever, a "smart" buyer would hock something or borrow the money and buy BOTH.
Prices will almost certainly skyrocket, and in a couple of years, you can likely double your money, certainly on the 60's nickel.
$600 is on the very low end of the scale for good Pythons these days, and it's really low for a 97% gun.
Going price is usually $700-$800 and up, to about $1000 RIGHT NOW.