"This would of course involve on occasion walking along the "shuttering jambs", from which they would occsionally fall off, in the bad old days before the HSE got a grip of the situation ... "

Spot on! Back in the days before Health and Safety went mad, projects like building a dam would mean building a near vertical wall with very little fall protection. It's not so bad for the first pour but as you continue up the fall get's higher and more likely loss of life. People generally climbed up the back of the shuttering Jamb instead of using a ladder(some still do). Labourers would have been climbing all over the Shutters when they were pouring the concrete. These shuttering frames were always made from timber by carpenters. Hence the name Shuttering jambs. In the same way that carpenters used to call door frames,door jambs.

Never in my twenty years as a shuttering carpenter have I heard anyone refer to getting a shutter stuck as a "Jam".