It's hard to tell without more info. If It's a Ford casting # - which it appears to be - the base # 43512 is related to the decklid, trunk, luggage compartment or that general area. The prefix BN suggests early 50s but I'm not sure which make/model. It's not in any of the books I have, they go back to '49. Might be Merc or Lincoln, I don't have a set of those books.
re: It is a trim piece very good shape maybe nos looks like tbird late 60's
Not likely. Is it stainless or aluminum? By the late 60s, most trim was aluminum and the id # if for a TBird would be something like C8SB....or C9SB.....etc. Body style 70 indicates that it was originally designed for a 2 door, probably a sedan or maybe a coupe, depending on the make.