Just a few of extra Skippers to add to David's pretty comprehensive description. The Carrera cased Skipper was ref: 7754 and was replaced by the pressure fit autavia cased version, case ref: 7764. Both used Valjoux's 7730 movement. This in turn was replaced by the ref: 73464, same 7730 movement but in a 70s plexi glass autavia case.

Heuer then added an automatic version ref:1564 with cal15 movement, this in turn was replaced by the ref:15640, same movement but housed in the heavier mineral glass Autavia case. The final version still had the 15640 model reference but was in the even heavier version of the mineral glass Autavia case and (I beleive) had a case reference 15064.

There was also a Skipper version of the Leonidas Easy Rider, sorry don't know the case ref. for that.

Jeff's 'On the Dash' web site has lots of catalogues for anyone wishing to research further.