The single action Invincible's were in a medium to bad state of repair, most of the piston seals were perished and there was rusting and pitting to some important parts that needed welding and regrinding for them to work as they should.
Whilst working on these gems we accidentally discovered the ingenious way that they were and can be made to hold open or none hold open.
http://www.chowbent-unitarian-chapel.org.uk/p/history.html