Hi Maestro,
First off, I'm not a deRosa historian.Just a long time owner that's observed changes to the frames over time-in particular from the mid 70's to the early 90's when the model range expanded.
Keep in mind also that the italian builders of the time were mostly smallish operations - regular model year differentation really didn't exist per se. Instead, the bikes evolved slowly and sometimes seemingly sporadically with the availabilty of components and fashion whims. Framesets were fabricated one at a time rather than as production runs and there was some leeway for customer/distributor input. My own frameset from 1979 was a semi custom order in a half size increment.
To my knowledge, DeRosa did not apply serial numbers to his creations until sometime *after* the move into the factory location (88?) .Prior to that time, any Alpha-numeric markings on the bottom bracket shell appear to be only code stampings to identify that particular btm bkt shell's tube angles and not dating /serial number info .
Fork crown selection and chroming details vary a great deal during the entire span of the 80's. The first sloping crowns were introduced with the limited edition Linea Aerodynamica's in 1980 and appear *most commonly* during the first half of that decade and then he *mostly* reverts back to the classic flat track crown style- no doubt with pressure from his distributors to ensure that DeRosa's have an unique aesthetic apart from the Merckx bikes of the time.
For 1986, he "officially" introduces his Microfusion Italiana "bridgeless" btm bkt shell.
1986 catalogue pic-
In 88, the trademark "Diamante" chainstays dissappear in favor of a more conventional oval crossection ( again returning in the 90's).
Also take notice of the seatstay cap logos in the 86 vs the 88 catalog pics. The earlier versions have the DeRosa script written lengthwise/longitudinally, whereas later versions have a vertical orientation.
1988 catalogue pic(35 anniversario)-
1988 colours( note both flat & sloping crown variations and tube style brake bridges)
In 89, the traditional tubular rear brake bridge is "officially" replaced by the cast unit.Flat fork crown becomes "standard"(No Am.)
89 Gita catalogue-
So as I see it, your lovely classic DeRosa Professional SLX would mostly likely appear to date from 89/90.
best,
k