Any chance you still have that, or I could a copy of that? That is something I would love to see. I'm also curious if the 100 sets was a typo, as Bicycling published that Jack Taylor claimed the order quantity was 1,000 circa 1976.

I'll write my own understanding of the Reynolds line-up based on Reynolds catalogs (presumably for UK builders). Conventions and stock for American builders, especially in the dead years for adult American cycling, may be different. Scroll down to the bottom if you don't care to read.

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In the 20's (before 531), thin Reynolds tubes were mostly 1/7/1. My impression is that with the introduction of 531, Reynolds sought to make even thinner tubes.

In the 1940's Reynolds lists (in nominal terms, 9=0.81, 7=0.71, 6=0.61, 5=0.56, 3=0.38) 8/5/8 TT, 9/6/9 DT and 8/5/ST as "Typical frame sets using ... 531, which would later be the same for 531c(competition). The DT/TT seem to be designed to have a ~1:45 wall/dia ratio, and the DT is just 9/8 the size of the TT. The 1/7/1 tubes are listed for "SIF bronze welded frames" (as opposed to lugged) but oddly enough it does not mention 1/7/1 downtubes. The SIF bronze recommendation is for a 9/6/9 downtube with a 1/7/1 top tube, and that's also what shows up as line items in the price list.

8/5/8 TT, 9/6/9 DT and 8/5/ST seems to continue as standard spec into the 50's, other grades of Reynolds seem to have other standard gauges listed, but butted 531 only shows 8/5/8 TT, 9/6/9 DT and 8/5/ST in the catalog and the addition of some lady's frame specialty seat tubes in the price list and seems to designate the thicker gauges for lower end tubing.

The first mention I see of a full set of 1/7/1 tubes being available as a standard catalog item is in 1970's (but I haven't found a 60's one, so they may have very well become standard in the 60's) catalogs. They also had a set of 1/7/1 extra long tubes for tall riders. The interesting thing is that for both 1970's catalogs, it only mentions 8/5/8 TT, 9/6/9 DT and 8/5/ST or full 1/7/1. No 9/6/9 TT/ST or 8/5/8 DT (except as part of SL), even though metric variants are mentioned. SL is definitely different from 531c. The catalog definitively states it is 7/5/7 TT, 8/5/8 DT and 7/5 ST. The equivalent to 531SL during the time 531c was produced would be 531Pro. The catalog states the tubes are only available as a complete box set, so I would find it curious if they sold the same tubes individually.

Later, 531c/competition was advertised as "classic," "thorough-bred" and "the yard-stick by which other materials are judged" and was 8/5/8 TT, 9/6/9 DT and 8/5/ST (except for a 1982 video which implies the DT is also 8/5/8 but Reynolds seems to have sometimes implied things that weren't the case). 531Pro was an updated 531SL (and confirmed by Reynolds' own documentation) with 7/5/7 TT, 8/5/8 DT and 7/5 ST.

Back to 1/7/1, it appears to me that 1/7/1 was an alternative spec to the standard for several decades, rather than being the standards with the lighter spec being the alternative. Here's what Reynolds has to say about 1/7/1 tubes. Reynolds seems to have been under the impression that 8/5/8 TT, 9/6/9 DT and 8/5/ST was standard, and 1/7/1 beefy. Reynolds used 1/7/1 on the DT and only the DT and an 8/5/8 TT, for 531ST(special tourist) as well as some some other beefed up non-main-triangle tubes. Also similar to later 531cs(club sport) primarily used by Trek, perhaps showing an American preference for heavier tubes. Later Reynolds introduced 531 Super Tourist (not the same as 531ST/special tourist) with a full 1/7/1 main triangle because Reynolds claimed touring bikes were being loaded up even more than before. Both the touring sets came with imperial fork blades for better clearance and spacing for cantilever brakes.

In some ways, I don't find it surprising Schwinn being Schwinn decided to use 1/7/1 for all their Paramount frames, and that might have had an effect on supply chains and other American builders. But I would find it odd that Europeans, especially UK builders would use 1/7/1 standard unless they anticipated abuse. Reynolds appears not to have thought of 1/7/1 tubes as standard, but as heavy weight tubes. Early on, 1/7/1 tubes were the tubes they made before 531, and 531 tubes were made to be thinner than that. Later on Reynolds seems to have thought of 1/7/1 tubes as tubes for exceptionally tall riders, or for loaded touring.

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In summary, I think it's pretty clear from catalogs that 531SL was different from 531c. The updated version of 531SL was 531Pro, and 531c was the label given to the updated version of Reynolds light-standard tubing. I don't know about the building practices of production factories, but Reynolds seems to have thought of 1/7/1 as heavy duty tubes, relegated to lower end steel in the early days of 531, and used for XXL frames and load bearing touring frames later on. This seems to be fairly consistent messaging by Reynolds through several decades.