In the US, electrical plug typically have holes, but not all of them. The electrical standards, which US manufacturers must abide by state that these holes are optional, and are intended to be determined by the manufacture to include them or not.
AFAIK, Apple has not released any details for each of their power adapters that pertain to these holes. As such, since Apple uses a third-party to make them, it may be possible that one manufacturer opted not to include them.
If you are sure you got this adapter from an Apple Authorized Reseller, then it should be an original Apple product.