I have owned several examples. I currently have one on document (VERY scarce!) with a PF Certificate. See below.
Certification is MANDATORY on these for resale, but a quick verification is to keep a copy of R145 ($2 third issue) handy, and for any R135 that you suspect might be R135a, place it on top of the R145. For the R135 to be vermilion the color should match the R145 exactly.
Because the orange of R135 varies greatly, you will frequently see darker reddish shades that when looked at individually seem like they "might be" vermilion, but when placed with the R145, you can tell they are not. Too much yellow pigment in the "vermilion" and not enough magenta.