For something of this nature, which might be common from the '74 models and forward, when used, if you find a '74 part number, if that part was changed or upgraded, it would be to the later model year's vehicle as a matter of course. But you have to have the Price Book to track those number changes. Of course, if the '74 part was upgraded in '76, with an appropriate part number, then the '76 number would be the "new number" until it might have been changed in later model years.
The other thing is that should the financial people have tightened their strings and demanded a little bit of cost to be removed from a particular model year vehicle (regardless of manufacturer), these changes were usually made pretty close to the new model production starting, hence, FEW real changes could be made. BUT what could change is small things the general public might not notice (like a less expensive seat fabric, a change in standard equipment in a small way, a few less chrome items, OR, in this case possibly, the deletion of those rubber splash shields in particular model years). Cars still looked the same until you started to notice minor things that were different or "gone".
As for the parts books, they had to be approved prior to the model year beginning, so they could get printed and distributed before show date. If the initial approved printing reflected "no seals", that's what they went with. If "seals" were later approved, it would have been too late to change the books, although a revision letter might be sent later (as was the case with service manuals or ordering information).
Sometimes, these "side issue" situations affect the ultimate accuracy of the parts literature. I've been through several situations where I had to go back one model year to get the correct parts.
Of course, this works only within the particular platform version's production life. In this case, '74-'78 C-body platforms.
CBODY67
