That MPC-era boxcar isn't part of the 6464 boxcar series; it's actually part of the 9700/9400 box car series (I have the same car somewhere in my collection). And there actually was a prototype boxcar that was painted up & lettered similar to that, celebrating the one millionth boxcar that Pullman-Standard made.
Strictly speaking, any Lionel-branded boxcar made after 1969 aren't considered "true" 6464 series boxcars, as that numbering convention was also the catalog # used by the Lionel Corporation from 1953 up until then. After that, when the Lionel Corporation got out of the toy train business and sold their tooling to General Mills/MPC heralding in the "modern era", the boxcars (and pretty much all Lionel-branded products in general) adopted a completely new catalog numbering convention that started with the 6-xxxx prefix that remains to this day. The first of the 6464-based boxcars were part of the 9400 series, eventually adpopting the 9700 series and then the 9400 series onwards; each revision due to running out of numbers for those numbering series. Even the "6464 series" boxcars made in the modern era have a completely different catalog # that doesn't match the car # like postwar did.
Having said that, I probably have a couple of postwar & modern era "6464" boxcar pictures lying around somewhere in addition to MPC & LTI era traditional boxcars based from the 6464 molds; I'll post them if I find them.
