Ruston Hornsby PB

I have recently been restoring my 1.5/2 HP ruston hornsby PB, and have this question for you - I noticed a small hole in the bottom of the
engine between the hopper drain tap and the carburettor. The hole extends in approximately 1 inch, and at the end appears to have something that can be pushed in (about 1/8th of an inch), when released it moves back out again (i am unable to see it but have used a bit of wire to push it). Can you tell me what this is and whether it should be like this.
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