I am afraid I am not familiar with the window mechanism on the 403, but on other cars I have encountered this problem where the channelling on each side of the glass wears (or moves deeper into the door frame) so that when the glass is in the lowest position, one or both edges slips out of channel.
The other situation I have encountered is where whatever on the winding mechanism stops the glass from being lowered beyond a certain point fails, the glass is then free to go lower than the designed lowest position and can come out of the channel.
In both cases, the way to reinsert it is to lower the window as far a possible and ensure that it locates properly in both channels as you raise it.
Hope this works,
George
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