check this thread: http://www.airforums.com/forums/f39/...tic-50799.html
I have the same trailer, with the same patterned end-cap. I wonder if it was unique to '73, because I haven't seen it mentioned on any other trailer. As such, most people just paint over their yellowed end-caps. But the pattern in the 73's sort of makes a fake textured look, that makes the cap blend in nicely with the textured vinyl wall covering.
for whatever reason, mine is not yellowed. must not have spent much time in the direct sun over the years.