Wool pile... Base appears to be 3/16, with a single fin.
A quick learner from Texas says:
January 23, 2016
I'have several windows in my house in similar condition, where the weather strip has worn down, or away. I cannot ascertain the height of the pile though. The windows are all Alenco 3710/3710F Windows, manufactured in 2000-2002. Not sure which the best part would be.
Paul from SWISCO responded:
January 26, 2016
Without knowing the height, it's hard to say for sure. You could consider our 58-021, though, which is 3/16" x 3/16". Could you get that to work?