B50 stretches more than B55 (BCY55). I pre-stretch my LB strings with 63 kg (140 lbs) weight, at least over night, usually a couple of days. I have not measured the difference, perhaps 50 % less. After pre-stretching B55 also sets faster to proper string height on my ca. 75 - 85 lbs bows, single loop on upper nock and bowyers hitch on lower nock.
Note: Polyester, the chemical name for these string materials, is the only allowed string material in IFAA’s Historic Bow classes. That’s why I’m stuck on these instead of using Fast Flight or other more modern less stretchy materials.