Slimslob, You are correct. Didn't have the documentation with me, and thought i had the right code. D037/D038 would be what i was thinking. With that said, I do remember now that those toners (black) don't fit in both series, there's a little tab or something on one or the other that prevents that (correct me if I'm wrong). I do know however, that you can put the wrong development unit in, and cause issues.