i am familiar with the open end spring bars. they are rare. i am sure i do not have a set,
you want to know how i make my own?????
i use a standard double shoulder pair. that is, one with a center piece and two extending pins
the small skinny piece that fits into the log hole and the hollow “follower” piece
i gently pull out the skinny solid piece
then gently ream the remaining piece
it will works perfectly.
you need to measure the interior from lug to lug. then the pins.
then get a few spring bars, two sets same size. you want the hollow moving piece to be say two mm smaller than the full inner width of lug to lug
so when solid pin is pulled out, the full size with the hollow sliding portion will be slightly smaller than full interior