Yes dev and fix one piece and just fix another. The latter will be pure white and it will be easy to see how much more grey than this piece is the potentially age-fogged stuff.

Benzotriazole can reduce age-fogging considerably so if the stuff is only light grey then it might be worth buying.

I don't think anybody can really predict how it will turn out. A test is necessary. For what it is worth I'd hazard a guess that only the MGIV might have no issues but even here and depending on storage conditions there can be no guarantee.

pentaxuser