two timing marks on cam?

[KFunk]

maybe this is a dumb question, but I've never gotten around to figuring it out.

On my cam, there is one mark that can be seen with the valve cover off, which is what I've always used for lining things up at TDC. But if I take my cam sprocket off, I can see another mark on the camshaft, which was hidden underneath the cam sprocket. At first I thought it also marked TDC, but I noticed it doesn't quite line up with the other mark. There's like a 1/2" difference between each mark.

Is this normal? I assumed the normally visible mark is the one to go by, and luckily the car runs fine. But now I'm just wondering, whats up with the different mark under the cam sprocket?

[TobyB]

sure it's a factory cam?

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