A square female receptacale plug? Don't remember that one, but then half the time I can't remember my name, the last guy working on it must have been a Chevy guy!
Note that there are several possible configurations of these galley plug scenarios thru the history of the FE production. The previous as I outlined is only what I might expect to discover in that (C4AE) block, but other FE's may/will exhibit other components; such as the 6 o'clock plug being also a 9/16" cup-plug later in production, then screw-in 1/4" NPT plugs being used throughout also, the loss of the first up/second down @ 12 o'clock as the oil relief valve is discontinued, and of course the addition of the the lifter oil galleries and resultant plugs both press-in and screw in, and then there's the "side-oiler" blocks
For a replacement plug, try Precision Oil Pumps, they offer a relief valve kit which includes the plug, or perhaps solely the plug could be had?
And, it would appear the cup-plugs have been inserted backwards (sounds more & more like a chevy guy
), and although there are plugs that are engineered to be inserted as such (example: FE cam plug) most often the design is intended otherwise , as it is the pressure applied to the convex side of the plug that forces the perimeter into the receiver boring wall within the block casting (in this case) providing resistance against to the plug being displaced.
So what do you think,.............do the plugs need to be changed?
Scott.