To me it is the plate & the u bolt. A quick web check defines a shackle as a metal link, typically U-shaped, closed by a bolt, used to secure a chain or rope to something.

Hi John,
I come from French Canada. Automobile terminology justifies the phrase that the British, the Americans, Canadians, Australians are peoples divided by a common language. Does calling a "trunk" a "boot" make the item clearer? The French around the world have very different words for the parts as well. The safest route is to call everything a "thingie" and post a picture.
L.
P.S. I have posted dictionaries, encyclopedias, translations into from UK to US and multiple languages for those who need them.