Well, I'm several months late here, but since you mentioned me by name, I'll chime in -- if only to credit your impeccable tastebuds!
When tasted at the 2006 Sampler, Gary signalled a special affinity toward a 1940/1948 W.L. Weller Special Reserve BIB I'd opened, which I'd found disappointingly ordinary. Fortunately, more than half the bottle came home with me, which I thoroughly enjoyed over the next few months. And, it was its chocolatey-ness which I found most endearing. It also appeared in a similar 8yo Weller BIB that Chuck Cowdery got the last taste from at last year's Sampler. His response? "Wow!"
In accordance with this thread, some time after opening, that same chocolate note was prominent in a 1953/1957 OGD BIB I opened at the 2007 Sampler as part of my 50th-birthday celebration (1957/2007). I've found it elsewhere in other contemporary bottlings to those mentioned.
So, yes, I concur with Gary that this is a profile defined in the Post-WWII years, whether by oak tree or grain bill (though it seems to affect both wheat and rye, which points to oak, doesn't it?).