Vee_Que;108949 wroteThey absorb low speed. Nothing else.
This. ^
The 74 model bumpers were designed to meet a US regulation which said they must be able to withstand a 5 MPH impact without damage. The 73 models in the US got different bumpers to the rest of the world, as they were nearly as big as the 74 bumpers.
The heavy part of 74+ so-called 'commando' bumpers isn't so much the huge bit of aluminum, but rather the rubbers and the support under the bumper rubbers.
It's worth noting that the front nose panels are different between pre-74 and 74 models. Up front, the 74 models are square; 73 and earlier have a larger opening to accommodate a pair of 'y' brackets. The rear design from 74 on changed to 240 style, where the bumper struts fitted into the redesigned rear chassis rails. 73 and earlier rear bumpers bolted onto flange brackets which were part of the lower beaver panel, which are absent from 74 140 / 164 and all 240s.
For Project Clyde (74 142), I am working out how to retrofit 73 model bumpers, just because I prefer that look.