What is the brass-ringed hole called on the the oil tank, next 2 the oil filter, on an N-reg Vauxhall Corsa?

I recently bought a second hand Vauxhall Corsa and after a long drive of around 200 miles, the car developed a problem and shuddered to a halt. All the oil had drained and it looked on closer inspection to have drained from a small brass-ringed hole at the bottom right hand side of the oil tank (not the sump), near the oil filter. Having looked at a neighbour's M-reg Corsa it looks like what's missing is a transparent 'plug'...





Until I can ascertain WHAT is actually missing or get it towed to a garage (limited as I work shifts) and have a professional (expensive) look at it i'm gonna just have a lump of metal at the end of my drive...What is the brass-ringed hole called on the the oil tank, next 2 the oil filter, on an N-reg Vauxhall Corsa?
Sight glass.





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