The image of the Gurian auction phaser keeps popping up, and I'd like to remind everyone that the phaser in that image has gone through some pretty horrible abuse,(I believe I remember seeing it in some sort of band image?) and it is at least 40+ years old. The abuse endured by the piece is further evidenced by the damage present on the fiberglass shells, as well as the missing midgrade turn knob. It's entirely possible that the vinyl crispy on that specific piece endured abuse which would have affected the luster of the reflective vinyl.
I believe that this is the case because if the crispy of this specific phaser is examined, some sort of thin coating of residue can be seen inside the diamond shaped cavities. This residue affects the shine of the vinyl, and is not present on other phasers.
In general, despite the fact that the midgrades were made after the heroes, I still think that the crispy material is vinyl, as the crispies on both midgrades and heroes exhibit similar types of wrinkling. I know that a wrinkle test has been performed, but I would like to point out the fact that it is entirely possible that the distortions in the travel bar backing were created when the phasers were still relatively new. Considering the backing of the Mylar, I do not think it would be a large leap to assume that the seemingly organic material hardened with age, thus making it harder to wrinkle now than when it had been first produced.