I thought about Megs too - but this is catalogue from 1990. Megs was pretty much dead by then, and the story was closing to an end. Although, Xaaron and Prime only appeared together on Cybertron...
I too remembered the Robot Master story - but Sparkplug and Buster were nowhere in that.
Germany and Austria had more of these made-up comic covers. I'll try to find an older pic I got in Austria back in 1990, which is only a blue/white sketch featuring OP battling the Predacons, with the Witwickys smiling dumbly in the background....
My best guess is that this ad was made not by Marvel but the company producing their comics in Germany, and they simply hired someone to draw a pic for them.
Btw, for those who don't speak German, the ad says above the 'Transformers' sign: "Galactic Adventure - Robots in fight for peace."
I'll also add another curiosity - here in Hungary, the comic's title was reduced to "Transformer" as the editor didn't like the plural form. After so many issues, it was finally dropped and we got "Transformers" at last.