Another data point. In an attempt to positively identify a Viking f&f from the mid-60s, I wrote (yeah, a letter - imagine that) to the V-CC ME for Viking and included some photos. He replied he thought it was one of two possibilities - a Conquest, fabricated from carbon steel tubing (i.e. gaspipe) or a Mileater, which is plain gauge 531 throughout. The way to tell was the weight. The Mileater frame and fork with headset installed would weigh in at about 6 lbs 14 oz, with the Conquest coming in a pound heavier.
My frame weighs 6 lb, 14oz (or thereabouts on an analogue scale). The headset, a TD Cross, would weigh what? Maybe half a pound? So we've got a 40-year-old frame made from plain gauge tubing coming in at under 6 1/2 lbs.
I have a way-out-of-timeline Tommasini Techno fabricated from Dedacciai SAT 14.5. The catalogue gives a weight for the frame alone of 1.65kg or 3.63 lbs. The fork (Nivacrom) is spec'd at 660g or 1.45 lbs. That's a total of 5.08 lbs. I make that less than 1.5 lbs difference. The Techno weighs just under 80% of the weight of the plain gauge Viking.
The 531 double-butted frames from the late '50s I have in house weigh less than the Viking - noticeably so just by comparison. I haven't weighed them because...well, I really don't care. But holding the Viking in one hand and a '58 Flying Scot Continental path in the other (both with headsets installed, Bayliss Wiley steel in the Scot), the Scot is noticeably lighter and may be closer in weight to the Techno than the Mileater.
Now there's tubing still lighter than the SAT 14.5 to be sure. And substituting one of those lovely carbon forks ( ;-) ) I'm sure will bring the weight down. But to half that of the Viking? And certainly not to half the Scot. Not if you're going to stay with steel.
I should note that these are small frames - 51 cm seat tubes. But I suspect the ratios would hold up the line.
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Phil Sieg
Knoxville, Tennessee