351 SVO block

I have a stroker Windsor based on the 351 SVO block. On the rear area of the block, it has a passage that leads into the coolant area. Just looking at it visually, it looks to be about a 7/16" diameter hole, but its not threaded, so I cant plug it with a common threaded plug. It appears to take some type of push in fitting, like the way an oil dipstick pushes into a block or timing cover.
Is this some type of coolant bypass or something??
What do I use to plug it with??
Can I tap it for threads and install a threaded plug??

What block do you have their are a number of variations

Well, I cant make out the rest of the casting numbers, but I have F5ZM-6010.
But I "think" its "W351".
It is a 9.5" deck and has a 2 bolt cap on #1, but the center 3 are splayed 4 bolt, and the last one is a 4 bolt with straight bolts.
Also, it is cast without an oil filter mounting pad, it has two threaded oil ports (inlet / outlet), but is still a wet sump design.
And on the rear passenger side of the block, it has this coolant source that either must be plugged, or plumbed into something.

Have you got a photo of the area, and is it on the rear china wall inside the lifter valley?

I had a w351 block in the past with the same hole you are talking about. It was a blind drilled hole went to nothing. I'm not sure what the purpose was for it. I ran the engine with no plug in it and had no leaks

Really?? I hope that is the case. I can see down in it with a flashlight and I can see there is some normal rust color like you would see in a coolant passage.
But then, yeah, why would the plug be missing??

https://2img.net/h/oi61.tinypic.com/t6a651.jpg
You can see it in this picture^^ its outside the China wall, next to the bellhousing face.

looks just like what I had. Not real sure what that was there for. You could put a knock in freeze plug or tap it for a pipe plug if you wanted.

Level the block in the engine stand and pour some liquid in that hole to see where it goes... If it goes nowhere, forget about it...

Well it does indeed lead into the coolant passage. Put water in the block, and it rose up and seeped out that hole.
Installed a press-in plug and all is well.