MustangForums.com

This link should help you out. It tells you how to find out which one in in yours.

http://www.americanmuscle.com/transm...dentifier.html

(The link has a picture)

Quote:

Mustang Transmission
When it comes to buying a Mustang clutch or a Mustang shifter, it is important to be able to identify which manual Mustang Transmission is installed in your car.
In order to identify what kind of Mustang transmission you have, you need to check the door tag on your drivers side door jam. If you look down to the bottom right of the tag, you should see an area labeled "TR." Underneath this label there are 5 letters and numbers. The only one that we are concerned with is the first alpha-numeric character which corresponds to your Mustang Transmission.
If you have a 6 as your first digit, you have five-speed manual, T-45 OD Mustang Transmission. This is the standard Mustang transmission that is found in 1996 to Mid 2001 Mustang GTs as well as 1996 to 1998 Mustang Cobras. If you have a 5 as your first digit, your car came with a T-5 OD Mustang transmission. Finally, if you have a K as the first digit in the "TR" code, you have a TR-3650. This Mustang Transmission is found in mid-2001 to 2004 Mustang GT, the 2003 and 2004 Mach1 and the 2001 Cobra. The 2005-2007 manual Mustang GT's also a T-3650.
Some of you may be wondering about the other 2-5 digits. They are related to the Mustang springs, front/rear which in this case, do not help you in identifying your Mustang transmission.