I think I would trust your own tests rather then a possible date from the factory

I my experience most people "Use" up any old supplies as a change over occurs if at all possible, I would think a gradual change over from Celluloid to actual Plastic would be pretty possible

Also for what it's worth keeping in mind one of the most insidious aspects of Cell Rot is it shows differently on many razors that looks more like bad care in the past rather then Cell Rot.. Your Plastic bag test is a great way to proceed..