why do you think the downgrade PB31IMG is unuseable? does the md5 not match? the things you are seeing are exactly what you should see in an RUU,but why are you extracting it? you need to leave it zipped up so the phone can flash it.
if youre looking for the clockwork image,its in its own seperate download,its not inside an HTC approved ROM. read carefully
1)download these files