Indeed, that's the trick, Carl. I suspect we're somewhat alike in that we would like to have the piece of paper that provides proof of acceptance. That would be a little difficult to find.
In truth, one of the reasons manufacturers put statements like this in their installation manuals is to give everybody an "out". Let's say, for instance, that ACK can no longer source the parts to make their antenna. Because they've put the weasel words in their approved installation manual they can almost instantly recommend a replacement antenna without having to go through a full re-test. They can say "oh, the RAMI AV-100 looks good, let's use that" and be back in business.
(BTW I've worked with products which were in exactly that position and had wished we had been smart enough to give ourselves a similar easy way out of a regulatory corner, and have in fact incorporated similar "outs" in other products. It's a hurting day when one has to look at re-doing qualification testing of an aviation appliance.)