lead [solder without the tin] was used to fill [smooth over] welded joints, as in the cab roof, with a paste flux
Not trying to quibble but it was solder, probably 60/40 or 70/30, as called for repeatedly throughout the manuals. Not sure what they used for flux in the 50s but in the 70's it was Ruby Fluid. Still available and the very best for tinning steel... in my experience.
