The WOSF were steel carrying conversions of the WOAX class open wagons (although some came via the WGG pool).  The most obvious change is that the doors were removed.

1987	WOAX 33016, 33045, 33046, 33077, 33069, 33089, 33110, 33121, 33134, 33139, 33145, 33162, 33173, 33224 to WOSF steel wagons WGG 33038, 33048, 33079, 33177, 33178, 33182, 33184, 33213, 33216 to WOSF WOSF 33045, 33046, 33077 to WOAX (although some doubt about 33045)
1988	WOSF 33089, 33184 to WOSX
1989	WOSF 33048 to WOSX
1990	WOSF 33139 to WOSX WOSF 33134, 33224 written off
1994	WOSF 33048, 33089, 33139, 33184 to RKZX
By June 2010 WOSF 33069, 33121, 33162, 33173, 33178, 33213, 33216 had been converted to AFBF flat wagons and the rest had gone.