The early M1903's have a different slide front end from the later ones. They have a barrel bushing and a recoil spring plug that is kept in place with a cross pin. The plug has an elongated hole so that it can be pushed in to allow the barrel bushing to turn so it can be removed. The bushing does not need to be removed to remove the slide, but it has to be removed to remove the barrel from the slide. (This was an early version of the idea later used in the 1911.)
Later, Colt did away with the barrel bushing, plug and cross pin, and went to a short plug that has no purpose except to fill in the hole. Why they didn't just eliminate the hole, I don't know; perhaps it was needed for some machining operation on the slide, like a pull through reaming.
Here is a picture from the early manual showing the bushing, the front of the plug and the cross pin.
Incidentally, most re-assembly instructions for that gun are wrong, or at best, unnecessary. All that is needed is to assemble the pistol except for the hammer. Then hold the pistol in the right hand as if firing it, squeeze the trigger and the grip safety at the same time, and insert the hammer into the gap at the top of the frame. Then insert the safety, which is also the hammer pin.
Last edited by James K; January 30, 2013 at 05:35 PM.