Originally Posted by FBinNY

I agree that it's not "likely" a crack, but it is a bit irregular for a production or mold mark of some kind. Given that, you probably want something better than internet assurances. So here's something that might give you the answer you want.

If it's a crack, then the likely cause is "hoop stress" form an overly tightened expansion plug. So, that's how you'll know. Inert the plug and tighten up to about what you normally do. Observe the crack for any hint of expansion. Even if it is a crack, it still might be OK because some of the resins makers use are very brittle and will crack before the structural fiber does, but I probably wouldn't ride it if I confirmed that it was a crack, except (possibly) on a city bike where I don't tress the bars much.