Congrats, nice score.
I worked there when this was made. I can confirm that everyone who worked there was highly motivated to do our best. Bill D never pressured anyone to go faster, the emphasis was on quality.
I forget when the larger frames went to OS (1-1/8") toptube, '90-'91 maybe? So yours (1" TT) is older than that, let's say late '80s. Tubing really is Prestige throughout, but some tubes were in thicker gauges commensurate with the size. I don't remember for sure but possibly as thick as 1.0/0.7 for the DT, 0.9/0.6 for the TT. It's still light for a 66, but not stupidlight. We had some good racers on them (by "them" I mean 66 cm Impulse) and even with the 1" TT, strong guys were OK with the amount of flex. Definitely a road bike though, not a sprinty crit bike. You already know this though since you had an Imp previously.
Rear stays and steerer are the same as smaller frames. Blades are the thicker of the two Prestige choices at 0.9 mm — same as Columbus SL, lighter than Reynolds 531. Plenty flexy for a comfy 66 cm road bike despite the stiff crown and short blades, short rake. Actually come to think of it chainstays might be one tenth thicker than the small frames got, but there weren't many choices in Prestige c-stays.
I'm guessing you'll find the steerer does
not have a TANGE stamp in it. If it does, that means Tange made the fork. D used Tange-built forks on early Impulses, but at some point we brought the fork production in-house. Nothing wrong with the Tange-built forks, they're nice, but a D-built is better IMHO. We got so good at making forks that we were able to sell batches of them to a couple other builders who didn't want to make their own. Great, we perfected the steel road bike fork just in time for it to be obsolete!
All the above is from fallible memory so no guarantees as to accuracy.
Ride and enjoy!
Mark B