The shop manual isn't a lot of help when it comes to more than the few words they use to describe the lower door seal that mounts to the cab floor lip for the lower door seal you're asking about.
This LMC Catalog Page
1947-55 (1st Series) Exterior Rubber Components provides a bit of an idea on this seal. Items 22 (Seal), 23 (Retainer strip) and 24 (screws for retainer strip) are what you seek and are secured to the top outer edge of the cab floor above the door step recess.
Note that there is a design change for 1947-49 vs 1950-55.1, so you'll have to decide which ones to use (if you haven't already gotten them).
Below is a pic of my 1951 3100 drivers door opening showing the edge of the cab floor and the long screwed down retaining strip that secures the lower door seal strip. Sorry for the dark view which doesn't really allow the actual weatherstrip piece to show up.
