- +Noah Friedman
is correct, the orientation of the 12:00 is really just a convention, particularly with wristwatches (early wristwatches are often different). 12 tends to be set up to be "up" when reading the watch. The difference between a hunter movement and an open face movement has nothing to do with the orientation of the numbers on the dial though - which can be any which way. A hunter movement has the 4th wheel (the second hand) 90 degrees from the stem. An open face movement has the 4th wheel 180 degrees from the stem.
If the dial is normal, it always puts 12:00 180 degrees from the seconds hand, so on a hunter watch the stem is thus at the three, and at the 12 on an open face movement, with a normal dial. What type of case the resulting dial/movement combination gets put in is also mix and match. A "side-winder" is a hunter movement, with a dial normal for a hunter case, in an open face case.
More examples here:http://elgintime.blogspot.com/search/label/Side-Winder
It is suspected that what case was selected when a watch was purchased probably had to do with price and availability, what the shop had on hand, as much as anything. Although many buyers preferred the more rugged, durable and generally masculine open face styles of cases and deliberately selected those, hence all the "side-winders".