89 XR4 150hp - help identifying a part

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I am trying to bring an old 89 xr4 150hp (S/N 0C184436) back to life following a voltage regulator fire nearly a decade ago. Wasn't sure if the fire was caused by poor circulation to the regulator so I figured I would go ahead and replace all components of the cooling system which includes the poppet valve.

So I pull the poppet valve cover off and someone has definitely replaced it in the past, but also behind the smaller side of the cover there is a plastic plunger piece that has an o-ring on it which looks like it's used to block water flow completely to that port back into the block, and I can't figure out if this piece is supposed to be in there or not. I've checked parts diagrams for the motor and I can't find this piece anywhere on there so I was hoping someone can identify it and let me know if it should be put back in or not.

I've included a picture of the piece I'm talking about.

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Anyone got any ideas on this?

[JOE54]

is that passage open into block like its been drilled out could have been part of kit to eliminate steam pocket at top of block, id put it back, monitor water psi and temp

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I don't think the block was drilled out, didn't look like it had been at least. There is an "old design" and "new design" of the poppet valve cover for my motor, but as far as I can tell the new design just has the line that runs to the top of the block. I was thinking it was part of some poppet valve conversion kit also but couldn't find anything of the sort anywhere online so I'm stumped.

I'll have to do as you suggested and monitor temps/psi with and without this plug in, see what happens.

Only thing I'm not sure I'll be able to check is if water is getting to the voltage reg. But the voltage reg I bought off ebay to replace the original that caught fire appears to be shot as it's not charging. So at this point I think what I'm going to do is mount the motor on the boat, put in a couple good batteries, and go run the motor for an hour or so to make sure everything is mechanically sound with it. Once that's done I'll bite the bullet and buy the CDI conversion kit to replace the stator and regulator to get away from that problem entirely.