DC - the battery is a OEM Toyota battery. I am not sure if it is the original or was replaced as part of the certified pre-owner program.
I raised the hood and tried to view the dates but could not see it.
1990LS400 - My main concern was the $165 price tag to replace the battery which seems a bit excessive. I checked online and saw replacement batteries at Autozone were around $115 so I assume the rest of the Lexus price would be for 15 minutes of labor.
Like you, my cheap tolerance level is high so I am not opposed to running the battery into the ground since I have towing as part of my insurance.
One thing you said has me wondering if I should just bite the bullet though - "And OEM Lexus/Toyota batteries are pretty darned good and have lasted longer than most of the aftermarket batteries, including the expensive ones, I've bought."