26a and 26b are both 4 rotor engines. a designates two side seals and b designates one side seal. it's a complicated system because the 12a would be a 12b if it followed those rules. so 26a and 26b are pretty much the same. i assume that if you built a 4 rotor, it would technically be a 26b. but i think (i may be wrong) that the original name of the engine that powered the 787b was "r26a."