By golly I think you're right. *I took a closer look. *I went to
Google images and looked up plywood. *It's definitely not plywood
lol. *I did the same thing with LVL and it looks more like that LVL.
Also after taking a closer look it appears that this board doesn't go
all the way up the way (I can see the small crack between it and the
floor. *So yes, this must be a LVL rim joist! Eureka! *This is making
much more sense now.

Thank you so much. *I've been banging my head on this one all day - Hide quoted text -

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WHEW!!! Your deck addition just got much easier, I was starting to
think the alternative approaches were looking like quite a bit of
work. Good luck, it should be pretty straight forward now.