Being flying Dirty Birdy's for several years now in SPA Pattern. My sig pic is a DB. What I've seen is some confusion over the Dirty Birdy and a later design Joe did that allowed the incorporation of a pipe which the Dirty Birdy did not. In any case one of the distinguishing features was the vertical fin LE which is concave in the DB but straight in the later design (I believe it was the UFO but I'm not sure on that). Anyway, I've seen several of these at least one of these at a contest listed as a Dirty Birdy which it is not.

Attached is a pic of an all-wood (not a kit) DB under construction for a friend. It will be using an OS .61 FX or an OS .91 Surpass. HE hasn't made up his mind yet.

No, the big MOKI is NOT going in this airplane! :-)