When the first issue of Dark Horse Comic's new Masters of the Universe & Teenage Mutant Ninja Turtles mashup comic series titled “Turtles of Grayskull” was revealed back in May, only three covers were officially announced. These covers were illustrated by artists Freddie Williams, Stan Sakai and Alexis Ziritt: Cover by Freddie Williams Cover by Stan Sakai Cover by Alexis Ziritt Well now it turns out that issue #1 of “Turtles of Grayskull” will have two more variant covers to choose from when it hits comic book shelves this September 25th: Retailer Exclusives! "Retailer Exclusives" are unique variants of comics that…