Very rarely you run into the over sized FWB Sport receivers.
I have seen some over the years. So these are over sized seals and
designed with a more forgiving grooved face.
Make sure the piston groove is clean and I like to tap
these into the piston with a mallet or hammer once
to make sure they seat well.
You buy these when the OEM seal or ours is to small.
I can not tell you what size your tube is. So try the standard seal first
unless you know you have issues.