Tri-ang Couplings

Original Tri-ang coupling (known as the Mk. 2a)

The Mk. 2b coupling had the addition of a "skid" to increase the weight and help to "drop" the arm.

Mk. 3 coupling was known as the "tension-lock",  when the vehicles were moving, because of the shape of the hook  it latched onto the bar on the other vehicle helping them to keep coupled.