I'm thinking this is the B & M box fitted to Rigid and Plunger models, as the usual box on B Models on S/A bikes is well covered on the Forum, being the standard gearbox type across A & B S/A Models' Ranges.
Mention of a link to the selector mechanism reflects this, the two designs are entirely different.
This earlier box is a masterpiece of belt and braces design and far too complicated for the job it does. Removal of the gear control plunger cap, spring and plunger should allow the selector shaft to rotate easily. This shaft is geared to the control quadrant, so wear or damaged teeth here can cause problems. Other thing to check is freedom of movement on the upper and lower ratchet pawls and associated components assembled onto what Drags call the gear change operating plate. On a long standing box, I reckon simple seizure of the selector mechanism is inevitable with the complexity of design. Selector forks move along the selector shaft as it rotates, as is now more common Japanese practice, so binding here will also prevent gear selection. Service Sheet 608 covers this box.
Swarfy