The Number Six and Seven Ponytail Vintage Barbie Doll sometimes are all called #6's and refer to what we are calling #7s as late #6s. There doesn't seem to be a consensus on which is correct, so I am presenting both and will explain the difference.
You will rarely see a doll for sale listed as a #7, usually they are all lumped in as #6s.
These dolls were the only ponytails to have Midge as a part of their markings.
Although all #6's have slightly fuller faces than previous ponytails, the #7s have a different body, in 1964 the word patented was added and they have a larger neck knob. The larger knob makes Barbie's face look even fuller. To me, it gives her an almost square jaw line.
So it is really impossible to know the difference without looking at the body and checking for the word patented. If it says Midge but not patented, you have a #6, if it says Midge and has the word patented, you have a #7 (may also be called a late 6).
The clue to if you have a 6 or 7 is coral (some call it watermelon) lips and nails. All ponytails before these had red lips and nails.
Speaking of lips, generally #6's have thinner lips, but there is a wide variety.
There are many more of these on the market than other ponytails, as they were mass produced from 1962 to 1964.
1962 - 1964
Different shades of Blonde, Brunette, Titian (red)
This is the first ponytail to have various blonde shades such as ash blonde and lemon blond. Instead of the previous black hair, this brunette is more brown.
Blue eyes. Brows were blonde, reddish or brown. Coral Lips and blush.
Black or Gold wire stand
(In 1963 added the word "Patented")
There was a variation with painted legs. The coral lip and blush colors varied a lot over the ears and many different variations can be found. Some are more red, some are more pink and some are more orange-y.
The #7 ponytail Barbie was replaced with the Swirl Ponytail doll in 1964.
Here are the current listings: