Thanks for all the responses! After watching Jeffrey Shapiro's presentation, I've realized why the search for elusive usages on cover is quite the challenge. The eBay
sales are astounding!
Is anyone familiar with the Presidential Stamp Series presentation portfolio published by the USPS in the early 1970s? It includes the entire 1938 Prexie series mounted on descriptive pages along with other presidential stamps through the 8c Lyndon B. Johnson issue. I am not sure if it was available for the general public or just presented to USPS executives.