The instruction sheet in the IMWX 1937 boxcar kit is copyright 1991. I first met Jerry Porter that year at the Denver National; NMRA convention ("Mountains of Fun in '91").IIRC he had just, or was about to, introduce the kit.
I don't think Dennis Lippert has chimed in yet on the C&BT Shops dates. When I discovered the rec.models.railroad newsgroup in 1992 Dennis was an active participant, and was doing assembly work for C&BT. I first became aware of C&BT's line of boxcar kitsĀ at a large train show at the fairgrounds in Pomona, CA, where Dick had test shots and lots of literature on display. My admittedly flawed memory says 1989 or 1990.