Prowin M4 Magazine

[TomFahey]

So I'm interested in using a HPA setup with my Prowin magazines, however, rather than drilling a hole and tapping the mag, I'd like to use the hole that's already there - the fill port. Removing the fill valve reveals a threaded hole - my question is what size is that thread? Unfortunately, I don't happen to have a set of callipers around, so I can't measure it myself, but it seems to be either 4/5mm I believe.

Cheers,
Tom