I think it is called a fan shroud that I just fabricated to go on the back of my track-T rad shell.  I noticed that the model 'A' built by the Tucci Brothers in New York had this on their car so I figured it must be something I'm supposed to have on mine so with the weather so accommodating lately, I got right to it.  Warm again tomorrow so I can paint it before winter sets in.  The shroud seems to tidy things up. I like it but no idea if I'm supposed to have one. If someone is thinking of doing it, it wasn't hard.  Took a while but worth the effort I think. Just make cardboard templates to fit, then cut and weld metal to match. Boy I sure get a lot of use out of old cereal boxes these days.