Kiz,
may depend on the RR.
which RR are You interested in ?
the 50' Converted Flats from cut down boxcars didn't start until
the early 80s,XTRA,C&NW,Southern,and others.
in the 50s some 40' flats were converted from boxcars.
DL&W cut down 40' Quad hoppers and built a new deck on the frame
of some of their 1955 era pig flats.
EL did use some Erie 50s converted to trailer service flats until CR
day if they were needed,but these were the 53' AAR type flats,not
50' flats.they did try to keep a supply of TT 85' to 89' flats in reserve,
but it wasn't always possible.
Spikre