I think you are right, it was assumed that one would use the TA rack. That one was the only one that had any meaningful distribution in the US. It was also assumed that one would attach the top of the bags to the bars with the provided straps, and that allowed the bag to work with a range of bar heights. When I got interested in cycling in the mid 70s, I viewed the canvas TA bag and its mounting system as a bit outmoded compared to the nylon bags by Eclipse, Kirkland and others. Soon enough, I discovered racing and had no use for any of them. It wasn’t until years later that I saw an Alex Singer with a custom carrier, properly sized bag and decaleur and it all made sense to me. I have since seen a number of TA bags mounted with decaleurs and it was a method that was assumed to be common knowledge in France, but I am certain that they were unaware of the complete lack of familiarity with that equipment in foreign markets.