Looks like a small block engine ID pad to me. It is found on the left front of the engine block just below the left side cylinder head. The "parting line" you see is the seam between the block and the head. For the stamping, I see "FW340 P31750570" which tells me F=1970, W=Windsor engine assembly plant, 340 CID, P=premium fuel, 3175=April 7, 1970 assembly date per 10,000 day calendar, and 0570=the number of the engine built at the plant that day.