I have the same shutter, it has a button that you push toward the rear of the shutter to hold it open, to focus. The button is about midway along the slot the cocking lever moves in. I found it more convenient to use "B" and a locking release as mine is on a recessed board.
Originally Posted by Klainmeister
No cable release socket? It must have been broken off or removed, this can be replaced. The 90mm Super Angulon is not a lens for press cameras!
To replace the shutter, you will have to match the threads and the spacing within a very very small tolerance. New shutter are almost unavailable, so the only source is cannibalising other lenses with all that entails - accurate measuring, CLA for the 'new" 40 year old shutter, aperture scales, etc.
A Copal "O" will have the same dimensions as a Compur "O", and so on.