@ Eric:
A friend just linked the following:
https://forums.maxperformanceinc.com...d.php?t=842270
That "Trans AM" tube looks like a dead ringer for the tubes shown on '74 GTO's earlier in this thread!
Then this (not from the linked thread):
That last image is for 1974 air cleaner part numbers;
It clearly shows the 1974 Trans Am (& SD455)
"F/8 4 B.C. RAM AIR" shaker base is the same one as the 1974 GTO
"X/8 4 B.C.-GTO"!
Although it's possible that the 1975+ shaker base was revised to use a different tube, for at least 1973 & 1974 the tube was specific to shakers!.
With this knowledge, I'd wager a that you can find one being reproduced!
So it looks like my collection of breather tubes does not include the 73-74 Trans Am breather tube - mystery solved!!
We would just need to verify that your car has the correct base - which I should be able to do easily enough;
If you have a latter base, then that means the "TA" breather tube was used for more years.
IIRC your '77 used the tube without the horizontal section - is that correct?