........... So much for 100% leak tested.
I would imagine any leak testing criteria is as a whole unit and not anything that checks ADBV retention between inlet and outlet of the media.
I believe it is a version of blowing air with the mouth on the threaded hole and seeing if it holds air. It probably consists of a threaded nipple connected to a compressed air source. The worker threads the filter on, turns the air on, watches if the gauge drops, then on to the next one. I don't know for sure but in the USA we couldn't pay someone to sit and do that all day. We design for no leak, test the design, then make it. If they are testing fast, which we know they are in China, they probably wouldn't see a small baseplate leak. As far as I know it does test the ADBV function. I also think moisture in the test air caused the rust. All IMO.