A couple of times in the Heinlein Journal, I've mentioned libraries which hold in their collections original Heinlein manuscripts or other primary source material.

I've just found that the University of Southern Mississippi holds correspondence between RAH and Martin Cole (12 letters from RAH, and 10 from Cole). See:. Dunno if this is anything interesting or not, and it may be available otherwise in the online Heinlein Archives or in the files at UC Santa Cruz.

The papers of Lurton Blassingame (RAH's literary agent) are at the University of Oregon, and include correspondence with RAH:

Papers of Robert LeFevre (prominent Libertarian), also at the University of Oregon, hold correspondence with RAH:

Texas A&M University holds substantial correspondence from RAH regarding his interactions with Shasta publishers and and with Playboy, regarding an interview/panel: . (Note that they also hold some of the letters of RAH's brother, Lawrence Heinlein: .)

The Theodore Cogswell papers at Syracuse University hold correspondence with RAH: .

The Blassingame papers are useless. I've checked them already, at least through the archivist.

Some of the others sound quite promising!

Robert