Good morning all.
What sort of relief valve are you referring to? It does have an O-ring seal but this could also just be a seal.
It's not for the charge cable unfortunately, the USB is the smallest end and this hole is about 3mm diameter.
It's got me rather curious indeed.
I recently purchased an olight javelot pro kit. It's come in (a rather nice) hard case which has an unusual threaded hole under the handle. There is a sealed plug inserted into the hole which can easily be undone and removed. It doesn't appear to serve any purpose, I'm wondering if anyone knows...