Alan,
I thought the same thing, unfortunately the three holes are offset to the right. (Look at Larry's [SCGuy] post somewhere above, it is real obvious on his photo) Would be cool to get ahold of whomever casts these things and tell them to sell you one before they drill out the vent hole. Or, just plug the hole on the right side with a nice smooth, polisheable aluminum plug and drill out the hole for the vent anywhere you want. I may yet do that.
Or, if you really want to get crazy, get a good aluminum welder to plug the hole with a slight overfill, and grind, sand, smooth and polish the plug. Voila, no more hole...
Bill