The Moss catalog seems to show that the TR6 uses "inverted compression" fittings, like the TR3A does. On the TR3A pump, the original compression sleeves were shaped differently and effectively thicker than modern sleeves are, with the result that the nuts sometimes bottom in the threads before fully crushing the sleeve.
Although the thread size is similar, screwing tapered pipe threads into a TR3 pump doesn't work very well. I suspect the same is true for the TR6.