I have never heard of this on this engine. Makes me wonder if someone has been into that engine before.

UPDATE: With heat indexes over 100 degrees the last two days, I wasn't able to work on it. But, I posted on the Buick Owner'* forum, and gained important info! That plug is access for a timing chain guide! Apparently, it'* not uncommon for the bolt to break, and the plastic guide to fall off! And, Dorman makes a special replacement plug, which has an extra nub on the end, which acts as the guide, and fits to the bolt hole, without having to try and get the broken end out! I'd hope that not even a year after having the chain replaced, that the guide failed! But at least I know it'* there now, and can check it! The BOF also had a link to the Dorman part, and a short video of how to install. Which involves, as expected, removing the air filter box, supporting the oil pan, and removing the engine mount. It'* supposed to only be 81 on Tuesday, so I'll ask for a PTO day so I can work on it!