It started life in 1990 as the DR-5L because David Reich. (DR) was the Founder/Designer/President of the Classe during it's pioneering years, hence the DR prefix.
David Reich left Classe in the early 90's (93?), and Classe took the DR prefix off their preamps and amps. They continued to sell them as 4L/5L/6L, until 1996 when they came out with the CP series 40/50/60.
So I would guess that the 5L was manufactured between 1993-1996 timeframe as Classe's mid-level entry. The CP-40 was manufactured between 1996-1998, as Classe's entry level preamp. It was superceeded by the CP-45 in 1998.
If you really want more exact dates, I've always found Classe customer service to be extrodinary. I haven't dealt with them since Armin left a couple years ago though.
I would e-mail Classe the serial number of the specific units in question, and they will probably be able to give you a manufacture date.
Cheers,
John