I'm thinking that Warren is saying that with the financial condition of Rootes especially during the transition of selling to Chrysler that money was tight for them and it's quite possible that it was a "use what you can get or what is in the parts bins" situation because some sources may have cut their credit lines or cut them off completely. That means there may have been running changes different from what should have been. There are cars in the states that were built with variances from what should have been.
While there were some variations towards the end the AMCO soft tops were not one of them. The soft top suppliers were still making them for other models post alpine/tiger and its not like rootes were shipping aftermarket usa parts to the UK for fitment.
I think Warrens comment is directed at replacement tops early in the cars life.
I could see dealers, especially those who had rootes as a secondary brand, fitting local patteen parts ... But in delivery the cara would have had the correct everflex tops