I don't think there are any intentional "assisted" openers. Most of them are by products of the action's cocking mechanism or the ejector work.
A few guns that come to mind are most Boss side-by-sides and any gun made on Baker's lightweight actions (aka the 12/20 action). Lancaster, Churchill, and a few other English makers used this action on some of their guns.
Bosses open before they've been fired because the ejectors push against the action face. I think the Baker-actioned guns also open easily before they've been fired - again because of the ejectors.
True self openers include Purdeys, H&H with the self-opening mechanism, a handful of Bosses made from about 1932-38, and, I think, Dicksons.
Some Euro makers also made true self openers on their own designs. I own a Defourny that opens/closes like a Purdey, but uses a different system to do it. I've also seen self-opening Francotte boxlocks.
OWD
