One source of basic information is the US Post Office Dept Daily Postal Bulletin
, which has been digitized and can be found on the website of the Postal Bulletin Consortium, found at http://www.uspostalbulletins.com/Home.aspx
This is essentially the resources that the USPS Office of the Historian would use when completing entries in Postmaster Finder.
The Daily Postal Bulletin
began publication March 1880, and exists today as a bi-weekly publication. Newer issues are posted on the USPS website, but the USPS does not really provide good records on post offices in them any longer.
I searched for Delrio, Washington for years between 1928 and 1943, and found the following entries. I have identified the entry for Delrio in red.
The "Vol49_Issue14733_19280706" identifies that the image which follows is from PB Issue 14733 published for July 6, 1928. Similarly for subsequent images.
Elizabeth Grant was appointed on July 2, 1928. Delrio Washington was a Fourth Class Post Office at the time.Vol49_Issue14733_19280706(Delrio WA Elizabeth Grant appt)
Delrio was upgraded to a Presidential Third Class post office on July 1, 1930. This occurred because the Postmaster salary for the previous year exceeded $1000 or $250 for four quarters of the year.Vol51_Issue15297_19300513(Delrio advanced to Pres class)
Elizabeth Grant is now reappointed as a Presidential Class Postmaster. Among other things, she would received a new Postmaster commission document from President Herbert Hoover rather than a Postmaster General.Vol52_Issue15512_19310129(Delrio now Pres Class, Elizabeth Grant reappointed)
However, the salary based on revenues did not hold up, and on April 23, 1933, the post office at Delrio was downgraded to a Fourth Class PO, and she was re-appointed as a Fourth Class Postmaster. This commission would have been signed by James A Farley.Vol54_Issue16158_19330502(Eliz Grand reappointed 4th class)
Finally, the post office was closed effective Sept 30, 1943.Vol64_Issue18661_19431026(DelRio WA closed Sept 30, 1943)
I would be curious to learn if your family has preserved any of the Postmaster Commissions which were presented to Elizabeth Grant. If you have them, please try and scan them and add them to this post.