I'm no De Rosa expert, but I don't think SL tubing was ever used in lower-end bikes. From that era I would say SLX frames were top of the line, followed by SL. I'm sure the real experts will chime in shortly.
Nice bike, BTW. I love my Professional.
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2010 AB T1X ** 2010 Cannondale SIX-5 ** 1988 Bottecchia Team Record ** 1989 Bianchi Brava ** 1987 Centurion Ironman Expert(2) ** 1985 DeRosa Professional SLX ** 1982 Colnago Super ** 1982 Basso Gap ** 198? Brian Rourke ** 1970 Raleigh Professional MK I ** 1952 Raleigh Sports