Brian in StL submitted this B.C. strip a long while ago, commenting: “When I read this initially, I had no idea. There are some suggestions in comments that might be right. The “Transformers” one in particular. I still don’t know.“
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Even after reading some of those comments, I’m just not sure whether this gag works at all: it seems rather contrived.
Comics I Don't Understand 
Definitely a Transformers reference. The leader of the Autobots acquired the title “Prime” (e.g., Optimus Prime). And the Transformers toy line slogan was “Robots in Disguise”.
And 2025’s Transformers toy line will be the Age of the Primes. (Transformers lore of the last 15 years or so has held that there were thirteen mythical Primes that used to run things before they warred amongst themselves and several died. Sometimes Optimus Prime is the reincarnation of one of them, sometimes the job is just named in honor of the Thirteen.)
I get that there’s a name connection with “Prime” but a prime number isn’t really a disguise for numerals. Certainly not any more than any other number.
Brian (3) Agreed. If anything, numerals are numbers disguised as letters. The joke is extremely forced.