Impact Air Rifle Telescopic Shroud / Silencer O Ring

[airsand]

So recently I won a few airguns in a storage auction here in NC and so forgive me because I am new to the sport but trying to learn more about how these actually function from an engineering stand point and eventually go on to use them this summer.

For the FX Impact... based on this diagram what number is it for the Telescopic Shroud / Silencer O Ring?.... is it B4 or A9... ::I am confused::

any help is appreciated