JQ, that serial # seems too high to be a '75...so I think you are right..likely a replacement.
The date the block was cast is stamped in the block just under the valve cover..The flame arrestor does a good job of blocking it, but you can see it..6digit number in MMDDYY format.
Don't confuse the fancy embossed number there too..I think that must be a casting number of some sort..
in this old picture you can make out my casting date above the arrestor 041976 - my serial is 17xxxx something..which on engines like mine where the flywheel is obscured, they also stamp the serial number on the flat area just under the oil fill.
