A 1972 MY with black metal guards seems odd since the rest of the trim was chrome.
As I know from nearly 40 years of dealing with 356s anything and everything is "ORIGINAL" on vintage Porsches! I have been told many times that part is "not original Porsche never made those". Even when I have the part in my hand and a photo of the car when brand new with the part on it.
That being said it does seem odd that a 1972 #836 (with the oil filler in fender) would have black metal guards. Since normally the other trim (horn grilles, trim around TS lenses etc ) is chrome. But again anything is possible...