bulgie 

Ha ha, no, not in those years, but In the '40s and '50s (maybe earlier too), smaller diameter blades at the bottom were in vogue, specifically for rough-pavement comfort. I have 531 blades as small as 12.2 mm at the bottom, and I'm pretty sure they made them down around 11 mm in days of yore. French constructeurs of high-end touring and rando forks seemed to favor the "pencil-thin" forks, curved the blades with a small radius form, and kept the bend all down near the bottom, specifically (I believe) to absorb more road shock. They even get Reynolds to make them with a rapid taper so more of the lower part was at the small diameter. That type of rake is back in fashion, but you can't get the really small diameters at the bottom anymore.

So, I went out to the shed to measure some blades. I didn't weight them though, sorry.

Name/Type Wall (top) Tip Dia.
531 wide oval 0.98 13.2
531 wide oval 1.03 13.3
753 (older, metric) 0.97 13.2
753 ('90s, inches) 1.02 13.0
653 1.00 13.2
Col. SL 0.85 12.5
Prestige 0.92 12.3
Excel 0.67 12.9
531 narrow oval 1.22 13.8
531 narrow oval 1.23 13.0
531 narrow oval 1.18 12.2
Kaisei “Toei” 0.96 13.2
Kaisei “Toei” 1.05 13.2
Prestige narrow oval 0.87 12.7
531 Tandem (non-OS) 1.40 13.1
'Jack Taylor’ tandem 1.40 13.5
Col. Tandem (OS) 1.20 13.0
Col. Max (original) 0.61 14.0

Why so much variation among supposedly identical blades, like the Kaisei or 531 examples? Some may be due to the inherently imprecise way I measured. I used high-quality Mitutoyo calipers, but that's not as good as a micrometer, especially the special micrometers made for tube wall thickness. Also most of the tubes had some odd shapes at the cut ends, not just a burr but sort of a rounded-over section, last mm or so. Hard to tell if it was bent, or metal removed or what. I tried best I could to avoid measuring on those areas. I deburred them as best I could, took repeated measurements at different places, and averaged the results (roughly, in my head, this ain't rocket science). I threw out any anomalous measurements that were way off, probably due to some localized lump or burr. I used a little pressure ("feel") on the calipers, same as it took to make the empty caliper read zero.

As to the original question, are 753 blades the same wall thickness as 531, I'm sure the answer is Yes, within the precision of my measuring method and/or the normal range of thicknesses due to manufacturing tolerance, and/or changes over the years. For example '70s 753 blades probably weren't drawn over the same mandrels as '90s 753 blades. My two sets of Kaisei blades were purchased a few years apart too.

Oh I said earlier I had a set of 531SL, but that was wrong. I have a box that says 531 SL on the outside, but with other stuff inside. I guess I just kept the box out of some sentimental attachment... I did find a 653 set that I forgot I had though. No surprise those measure the same as 531, since Reynolds always said they are 531 blades.

Mark B