These sport rider models all (I think) have two rear zip vents low down on the sides on the back of the jacket to deal with the airflow ‘exhaust’. There a similar arrangement on many of the more sporty bates jackets and I guess some other brands too. They seem to work surprisingly well. I think only the Vanson hurricane model has perforations on the back as well as the front (the entire jacket is made from perforated leather, but there’s no venting adjustment of course). The heavyweight nature of vanson leather jackets, which you’re of course far more familiar with than most given your wonderful collection, and the close fitting nature of these sport rider models probably helps with minimising ballooning too. Granted it’s not perforated, but I’ve never had any real issues with my CSRX ballooning even on very hot days touring abroad on motorways with the (aftermarket added) chest zips open, the main zip at half mast and doing illegal speeds t keep cool on an unfaired bike. I guess being canted over when riding like this helps somewhat, as does having to slow down after a while when my neck muscles can’t take trying to keep my head on my shoulders any longer!