They built about 200 '05 Mustangs in June - July in a pre-production pilot run.
Several hundred real production 05 Stangs were also built in August and the 1st week of September. From what I heard anywhere from 500 to 1000 of them were built prior to Sept 7.
For the '05 Stang, the Sept 7 Job 1 date is not the real start of production. It is when they say "all systems are go, time to bring the production like up to full speed." With a brand new from the ground up model that is being built on a new production line, like the '05 Stang, it takes quite a while to work out all the kinks in the production line.
The same sort of thing will not happen when they change over to 2006 Mustangs because there will be only minor changes from the '05 - '06 so it will be a rapid switchover to the '06s.