Here is the same CST, this time with a primary of 4 strands of 0.3mm ECW......(4 strands used purely to cover more "space" in the bobbin and so couple better to secondary).....the 4 x 0.4mm strands were easier to thread through & round the bobbin, but were far harder to solder terminate to the pads.....the TEX-ELZ, being solderable, was a lot easier to solder terminate to the pads as its jacket melted off easier than ECW lacquer.
I think there is a newer type of TIW with really easy solderable jacket, but its more expensive.