Hi,
This is the breather hole for the Bellville Spring part of the regulator. As stated by bchannell above, air leaking from here means one of three o rings on the regulator are bad. Below is a video from Sub12Airgunners on rebuilding the regulator. They do it for an Impact, but the process is identical.
Unfortunately for you one of the three o rings it could be is the one in the receiver block body (#3 in cut-away drawing below - 10x1.5 NBR90). Jeff (of Sub12Airgunners) shows how to get this out, but it is a PIA to do. I bought a set of Jeweler's glasses (with light) and found that it is way easier to replace when you can see it. Also got some brass picking tools. You absolutely don't want to scratch the inside of the block.
The other two it could be the one on the end of the piston (#1 below - 4x1.5 NBR70) or the one on the inside of the regulator body (#2 below - also a 4x1.5 NBR 70) both are easy to replace. I would replace both of those first and put the reg back in to see if it still leaks before going after the PIA one.
Good luck and try to enjoy getting intimately familiar with your expensive toy.
Cheers,
Greg
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