Gear tube - help


i need to buy a Gear Tube for my Vespa 50s (1975), and i founded 3 options on SIP site:

Gear tube V50S/SR, 4-gear ---> l=82mm, Ø=24mm 
Gear tube V50 Special V5B3T ---> l=111mm, Ø=24mm 
Gear tube V50 '77-, 4-gear ---> l=96mm, Ø=24mm 

the length they refer, is the side of the fist?

it was told to my that the 50s has  110mm on the fist side and 96mm on the other side.


Thank´s for the help!