Part of the confusion stems from the fact that Reynolds tube thickness, even the French or metric-specified diameters, was always drawn to be a certain "gauge", and some British Wire Gauge numbers don't match up well with decimal millimeters. In fact, gauge numbers aren't even linear - the jump from one gauge to the next varies. Note for example how well 19, 20, 21, 22 and 23 gauge line up with 1.0, 0.9, 0.8, 0.7 and 0.6 mm respectively - but then 24 gauge (what the French call 5/10) is not .5 as expected, it's 0.56 (or closer to .6 than to .5)
I suggest viewing the following chart in a mono-spaced font to make the columns line up.
British Imperial Standard Wire Gauge (SWG): _________________________________________________________________
SWG metric Usage example _________________________________________________________________ 28 0.376 = thinnest 753 unbutted section (3/10 in French) 26 0.457 = regular 753 unbutted section (4/10) 24 0.559 = "normal light" 531 TT unbutted section (5/10) 23 0.610 = "normal light" 531 DT unbutted section (6/10) 22 0.711 = "normal heavy" 531 DT unbutted section (7/10) 21 0.813 = "normal light" 531 TT butted ends (8/10) 20 0.914 = "normal light" 531 DT butted ends (9/10) 19 1.016 = "normal heavy" 531 DT butted ends (10/10) 18 1.219 = heaviest Reynolds butt I know of (12/10)
"Normal light" and "normal heavy" were not Reynolds terms, that's just me.
Tabulated another way:
Thinnest 753 tubes: 22/28 gauge or 7/3/7 (loosely speaking; common usage) Marketed as "531 SL": 22/24 7/5/7 "normal light" 531 TT: 21/24 8/5/8 "normal light" 531 DT: 20/23 9/6/9 "normal heavy" 531 DT: 19/22 10/7/10
(notice the middle, unbutted part of the "normal light" 531 TT is the same as 531 SL, only the butts are heavier)
And remember, the "5/10" tubes are really closer to .6 mm in the middle, 4/10 is closer to .5, and the 3/10 is really closer to .4 mm.
In comparison, the Tange Prestige 3/10 tubes really were 0.3 mm in the unbutt (yes I did cut them to measure the wall thickness in the middle) - also only .6 at the butt compared to .7 for 753. A little out of the CR time frame though. Butt then there was Ishiwata Alpha, which was out in CR-time, and it came as light as .6/.35/.6, a bit lighter than any 753 or 531 ever made - I think.
The 8/5/8 top tube, 8/5 single butted seat tube, and 9/6/9 downtube was often referred to (in the USA anyway) as the "Club pattern". Sounds like Colnago should have used it! (Ba-dump-bump) It was probably the most common set sold in the US, though higher-volume builders didn't buy tube sets per se; they got one box full of nothing but top tubes, another with downtubes etc. These could be ordered separately in any gauge, and many builders mixed the tubes differently, but still there is some validity to this Club pattern set due to it's extreme popularity.
By contrast, Columbus SL has all three main tubes the same thicknesses: 9/6/9. And SP is also the same all 'round the triangle, it being 10/7/10. Though some builders put an SP DT in an otherwise SL frame, or SP DT and chainstays, still the most common scenario was to stick with the sets as ordained by the tube manufacturers.
Mark "don't even get me started on rear triangles" Bulgier Seattle WA USA