Originally Posted by T-Mar

Here you go! Note that 21 gauge = 0.8mm, 22 gauge = 0.7mm & 24 gauge = 0.5mm. Also note the reference to it being thinner than 531 but not as thin as 753. Introduced in 1975, it is immediately recognized by a red coloured "531" on the tubing label, as opposed to the standard green. IIRC, it was used on Thevenet's TDF winning Peugeot PY10 in 1975 & 1977.