I had always been aware that 6.9s had engine mounts that carried a different part number to those used for the 450 model range fitted with the M116 and M117 engine range, and there was of course quite a difference in cost between the 450 and the 6.9 units - in fact, the price is probably double nowadays, with 6.9 units retailing from the dealers for about $320.00 each.
It's been a well known fact that quite often 450 units are used instead of the proper ones - obviously on account of cost...