It looks like Brad is referring to the " . . . metal trim piece (3-actual pieces per side) that secures the weatherseal to the door frame . . . " and those windlace retainers were used from 1947 through mid-1949.

The late 49 through 55-1st channels are available (they were/are spot-welded to the cab).

As BIG CHEVY 3600 suggested, your best bet is to place a Want Ad in the Swap Meet.