Bob,
I think that that angle iron is spot welded to the bed side. Somebody will correct me if I am wrong but the only truck I can examine this minute is my '51 and it is attached that way. You may not be able to detach it. Look for spot welds along the outside of the bed side. Another possibility is that it is some sort of anti-squeak material. I would not try to seal it, you may just trap moisture if not a complete seal. My 51 and my 37 are not sealed in this area.

Kent


1937 Chevy 1/2 ton
1942 Chevy 1/2 ton
1947 Diamond T Model 509
1951 Chevy 1/2 ton
1950 Chevy COE Model 5700 ~ "Barney" ~ And more pix