Tim - Here's a screenshot from the 1957 Chevy Master Parts Book of the 1951-53 truck door (actually includes '54 and '55.1 1st Series AD trucks) that shows the window and channel as a solid un-notched piece.
Not sure what you've got their, but you can replace it with reproduction pieces from many of our usual vendors. A cautionary note - if that was changed it could be your regulator may have been changed too???
Suggest your do further investigation and perhaps provide some pictures of your door and regulator so we can offer more info.
