I had to look that up.Are b55 and b50 the same material? Thanks for putting up with all the elementary level question, ha.
Both are Dacron but (from what I read) B55 has slightly less stretch than 50, and the 55 is a little thicker. There doesn't seem to be a lot of critical differences in the 2 materials. One guy said he likes the 55 texture for twisting flemish style strings.