The effects loops started out being a serial effects loop (the one you denoted with only send knob), but that's only on about the first 500 - 750 produced. I don't think there should be any like that with a serial above 1000. I'd have to double check. After that, they switched to the parallel effects loop from then on out. I would honestly stray away from the really early Recto's in that range back from '92 if you really like a good clean channel. Aside from the loop, those first run of about 750 or so also have an almost non-existant clean channel. It's ok, but not as good as the cleans after they made the switch.
From there on out, you have what I consider to be the best of the bunch, the late '93 - '95 or so, and some of the early '96 models. They have excellent bite and attack IMO, contrary to the post above, and I absolutely love the clean channel in it with a good set of JJ's in the preamp. I can get some very nice clean sounds from the amp, and probably my favorite on a high gain amp.