I believe 1971 is a really bit too early for 40' COFC. The ISO standard dimensions for containers were only firmly established a year earlier in 1970.
I have a fairly extensive collection of pictures of TOFC and COFC trains, and although it's by no means definitive, it certainly speaks to when things became 'popular'. In early 70's photos that are not close up it can be hard to tell if you're looking at containers or Flexi-Vans. The earliest picture I have that definitively shows containers is 1973, and they are the 35' Sealand containers (see Wheels of Time for model kit) on the Western Pacific. The early Sealand containers were 35' not 40.'
The earliest photos I have that definitely show ISO 40' containers are from 1974. They are of UP and ATSF trains and include some long blocks of 89' flatcars all with containers. I would say that's probably the year that COFC became 'popular'.
Other early companies that seem to have been common in those early days include Maersk (old design, like
this), Showa, Japan Line, and possibly Mistui OSK. Also a fair number of orange and brown containers without large logos whose ownership is harder to determine.
