Unfortunately, that's because 74/75 TAs are the step-children of the make. Oh there's a few people who give the 74 its due, but that's because you COULD get a 455 SD and if it doesn't have at least a 455 then it's worthless. The 75s are true orphans because the 455 was gone (except for a very few 455 station wagon engines at the end of the run) and becauase the TH400 wasn't used (needed more room for the catalytic converter, so shifted to the TH350).
To directly answer your question, I think that is actually the bumper. I've heard it called the bumper rub strip also. Brett, JasonD, or Joe Bays could tell you for sure.
Rick ---> who feels lonely as the owner of a 400/4 speed 75 TA at times...