This older series of OS needles which was the most common used on many different OS engines has no 'O' ring.

You may also want to note all FP's (which are very user friendly engines) utilise bleeder carbs for mid range and that horizontal screw in the front of the carb is the mid range adjustment. This screw intrudes into that visable bleed hole on the front of the carb.

What is important to remember is that this screw works exactly the opposite of the now common double needle type mid ranges. That is this screw is turned out to lean the mid range.

If your engine runs well then do not stress at what you may conceve as a leak.

John