Yes, and as I said in beamjockey's previous discussion about this, Olaf Stapledon actually riffed in 1930 on Dirac's speculation that the electron's antiparticle was the proton, and wrote about a ray weapon in Last and First Men that made them annihilate to produce total conversion of matter (the neutron hadn't been discovered yet). That's not quite antimatter as we know it, but it's got to be the earliest reference to something like it that I can think of. Still, it's not in the mainstream of SF treatment of antimatter the way Jack Williamson's stories were.