D. Scott Chatfield
The 50-foot LCs were converted in the late '70s. The cars converted in the late '60s were all 40-footers AFAIK. A look in the April 1972 ORER appears to confirm this.
The former NS 50-foot LCs predate the Southern's. Don't know if they were built for kaolin or phosphate. I know they were used for kaolin after the Southern bought the NS.
The ORER definition of an LC is "a house car with roof hatches and side doors. May also have end doors."
I recall looking in my 1964 ORER years ago and finding very few LCs, so they are definitely a post-1960 thing.