Cast iron clawfoot tub diverter faucet riser stem question: what is this called?
http://www.vintagetub.com/bathroom/tub-faucets/tub-wall-mounted-faucets/randolph-morris-clawfoot-tub-faucet-with-handshower-and-porcelain-lever-handles-rm155bn-s.html
http://www.vintagetub.com/bathroom/tub-faucets/strom-plumbing-tub-faucet-with-diverter-for-riser-connection-p0006c-s.html
http://www.clawfootalley.com/handshower3.html

It is your Diverter valve, and the "neck" is your male (IPS?) shower riser interface.

Out of curiosity, if this valve is supposed to mount to a slanted wall of a tub, you might be able to find some hardware parts to allow you to adapt the "standard 90" units you are finding to mount to the tub with a tapered/slanted washer type of hardware to work with the 90.

Also, you could use an intermediate riser or custom  riser with a bend in it to meet your shower head (assuming you aren't on board with using flex tubing).

From what I read, it does seem like identifying the current faucet make is a good approach since apparently there is no standard defined for what angle the tub manufacturer used, so you will have to special order something, or order what you want and pay a shop to customize it to your needs.