The ore car story keeps getting better and better... if you equate "more complex" with "better."
I had forgotten that Wingard's book "The Collector's Guide to N Scale" does include a section on the Rivarossi releases. And... surprise!
Three of the four cars that we have been discussing-- the LS&I, Great Northern and Soo Line-- are shown with catalog number 9399 (all three of them) and the note, "The following ore cars are listed under Atlas."
And... double surprise!
There is a whole series of ore cars that <are> listed under the Rivarossi brand name that are not part of the Atlas lineup, or at least they weren't during the A1G timeframe. Here they are (I hope Yahoo! doesn't mess up the formatting too much):
6001A - ATSF 5995
6001B - C&NW 538647
6001C - MILW 54058
6001D - Conrail 29540
6001E - UP 4500
6001F - SP 4836
6001G - GN 93715
6001H - PRR 125291
6001J - BN 6774
6001K - Chessie 5987
6001L - RI (Rock?) 58049
6001M - Southern 23450
6001U - Undecorated (black)
There's no "6001I" listed, I guess it was thought that it would be too confusing with "1" or "J". The Chessie System ore car happens to be the example pictured in Wingard's book and there are no subsidiary reporting marks. The photo is far too small to make exact comparisons, but it does seem to be the same body style as the Atlas car from what I can see.
The practice of using a single stock number with a distinct letter to distinguish releases was also maintained by Con-Cor during the same time. We know that Rivarossi supplied Con-Cor with some product, I believe after the Atlas relationship ended but not sure of the exact dates. I think that that practice may be a clue here.
So now what can we conclude? Probably, "more research is required"...
Cheers,
George
