Well actually, don't know where I saw it recently, but there was a post (on some other site) that showed it as part of a patent whose purpose was to act as a gas relief port.
The U.S. O3A3 doesn't have it, and neither does the P17 and both use chargers.

Always thought it was a charger cutout but makes more sense that it is a relief port for the left side of the action.

Not claiming this as my idea, read it elsewhere.

Cheers

Tom