Thats the clip but with the leather 5 point (i think its called)...
I'd like one as easier to detach at dressage after the initial 'settle', but also for when were jumping as i'd like to use my stud girth (when i have the studs in) which when I bought didn't occur that I wouldn't be able to thread through my martingale!
Just thread a dog clip through your current martingale! If it's not quite designed for that - strap too long - get your local saddler to shorten it and add the dog clip.
plas equestrian will make you a leather one (endurance riders use clips not buckles, & plas also do leather work), or synthetic look at zilco, pioneer harness or performance equestrian