I've made both Flemish and endless bowstrings for all my trad longbows and recurves, for the last 70 years (yep, no foolin').
Spinning an endless bowstring requires a jig and the length is preset by the jig arms.
Twisting a Flemish bowstring requires no jig, but setting the length and second loop requires back twisting and a bit of luck setting a proper loop-to-loop finished string length.
It's much easier and way faster to twist up a Flemish bowstring with just one top limb loop and tie the bottom limb loop with a bowyer's knot. Just make the length of the bowstring about a 8" to 10" longer than the bow's AMO length. The non-loop end needs to be Flemish braided tightly for that extra length. I highly suspect this was done way back in dayze of yore as one string can be used on any number of bows by simply adjusting the bowyer's knot.
The bowyer's knot works well enuf ... but ... there's an even better/stronger knot - the Japanese Tsuru knot - and that's all I've been using for well over a year.
tsuruknot.gif