I'm not totally positive, but I don't think there are any internal differences between a '79 and an '84 benjamin. As you've pointed out, starting in '78, the benjamins received the sheridan pump rod, cup, and valve. In '79, the sheridan received the benjamin receiver. In '84, both the benjamins and the sheridans were drilled and tapped for the williams peep sight, and at that time the benjamin got a taller front sight which was necessary for the williams peep. I think sometimes, people say the 'Racine' benjamins are the best because; A. you know it has the upgraded sheridan internals (even though technically a few years earlier also had this), and B. they will be drilled and tapped for the peep sight which is dramatically better than the rear sight.