They were all aluminum decks for the 2 model years. This model was specifically used to "promote" the F series engine.
Can't remember where I read it, as it's been awhile but OMC had a slow start in sales with the F series when it debuted in '78. With slight deck changes, the new engine had quite a few changes. Try and google it, and you may come up with the info. The pre-compliant F has always been my favorite in terms of reliability, performance and over all ease of maintenance.What else did they promote with these mowers? I've had no luck finding out on the interwebs.