When Barbie was first introduced in the last year of the 1950's she was a young woman. The teenager had not been "invented" yet. The early dolls look far much older than the teen girl she is supposed to be, but that was the way of the time. She had molded eyeslashes and rooted hair in a ponytail. The rooting pattern makes people wonder what happened to her if the ponytail ever got loose. The hair was rooted around the sculp and not over all of the head. Same goes for the Swirl ponytail that was released in the mid 1960's. The Bubble Cut Barbie however, had rooted hair all over.

The first sculpt has a few adjustments over the years, but I have them all on the same page here, Ponytails, Swirls, Bubble Cuts, repros and Silkstones. This is merely due to the fact that faces are "the same" - yet again with minor changes, but that might be the plastic och molds etc. They all have the "same" face with the molded eyelashes. 

As for markings on vintage Barbie dolls, the heads are not marked until the American Girl (Bendable legs) and Fashion Queen Barbie were released in the mid 60's. As to why that was, I can only speculate, but there were a lot of clone dolls appearing with the same ponytail or swirl style Barbie had. Might have something to do with that fact. The first heads with the markings had these inside the rim of the neck. The visible marking, as the one below came with the repros and Silkstone dolls.
 

Barbie original
Copyright: 1958

#850 Ponytail Barbie

#850 Ponytail Barbie

#850 Swirl Ponytail Barbie

#850 Swirl Ponytail Barbie

#850 Bubble Cut Barbie

#850 Bubble Cut Barbie

#850 Bubble Cut Barbie

 Reproduction

The reproduction versions were of course, also made with this sculpt. Wouldn't be much of a repro otherwise, right?

#13675, Busy Gal Barbie repro, 1995

N4974, The Original Teenage Fashion Model Barbie, 2009
Part of the 50th Anniversary series

Fashion Model/Silkstone

The Silkstone Barbie dolls are made with the original "molded eyelashes" sculpt. It is kind of back-to-basics but these dolls are made from a special kind of plastic, with a more porcelain feel to it. It's a heavy plastic and the dolls are posed in a model kind of way. The first dolls in the series was lingerie models but Mattel had to dress them up eventually. Too racy to have dolls in silk and lace lingeries... ('eye roll')
These dolls have come in several skin tones even if they were "porcelain" white from the beginning. I love the black dolls!

#29654 Dusk to Dawn Barbie, 2001

#56120 The Lingerie Barbie #5, 2002

B3431 Chinoiserie Red Moon Barbie, 2004
sorry, for the out-of-focus shot

Copies are the biggest form of flattery. The old original Barbie sculpt has been made into clone dolls as well. I stumbled across a black doll version the other day and didn't realize until I had her on hand, she actually was made with the original 1958 head - as well as body. Well, a clone version thereof.