First off here is a parts list;
https://www.anest-iwata.com.au/spare-part-guides/airbrush-spare-parts/hp.trn1-spare-parts-.pdf
I haven’t service that model Iwata, but my understanding is the parts are made by Sparmax Airbrush co. It looks like you still need to remove the entire internal parts of the barrel and use a jewelers “flat tip” screw driver to tighten the packing washer. I usually turn the nut an 1/8th of a turn clock-wise at a time… reassemble and check it. Note: the needle should have a slight drag when you pull the needle back and forth. I check this first before tightening the nut. These packing washers are rubber and usually need adjusting over time. Hope that helps you..
Ken

The packing on those models would be teflon ,not many if any manufacturers use rubber for the needle packings these days except maybe some very cheap Chinese junk ,they will adjust like most standard airbrushes