I've asked the same question here the forum, but haven't had any luck with additional info either. But here's what I know...
The club started in 1974. You're correct -- Bill Harris founded it, reportedly after he was unhappy with the direction TTOS was headed at the time.
I have a few of the club's "Toy Topics" magazines (I'm still missing a bunch), and the last issue I've been able to find is July 1978. I'm not sure how much longer the club lasted beyond that date, but I don't believe it was too much longer.
I've found several different gondola variations for the 3rd Convention in 1976, and also a "banquet" hopper for 1976. The gondolas continued for the 4th Convention in 1977, and then the caboose for the 5th Convention in 1978. Bill Harris had a flea market space/train store at the time, and he used all the 027 gondolas and other cars he couldn't otherwise sell for the overstamped cars. I'd love to know if other cars exist.
From what I've seen, they typically go for around $25 each on eBay, with the 1976 hopper bringing more because it's overstamped on a postwar olive drab hopper. (Apparently that was a near-worthless car in the 1970's!) I've seen that caboose with the crazy price on eBay as well, and it's just that -- crazy. And on top of that, the price has gone up several thousand dollars since it was first listed several years ago.
I'd love to hear about what ITBA cars you recently acquired.
TRW