This is the factory "lifting eye" or "hook", which is the front attachment point for engine removal. It attaches on the passenger side of the cylinder head in front of the first spark plug using two bolts and washers.
Datsun 280Z, 280ZX
Datsun S30, S130
1975, 1976, 1977, 1978, 1979, 1980, 1981, 1982, 1983