Dear Ms. Davis-Long,
Thank you for posting your request on History Hub!
We searched the National Archives Catalog and located Reports of Site Locations, 1837 - 1950 (M1126) in the Records of the Post Office Department (Record Group 28) that includes the file Maryland: Frederick - Kent (Roll 268). We also located Records of Appointment of Postmasters and the Establishment of Post Offices, 7/1/1931 - 10/31/1971 (M841) in Record Group 28 that includes two files of Postmaster Appointments for Frederick County, Maryland (Rolls 132 & 55). All of these files have been digitized and may be viewed online using the Catalog. If you have any questions about these records or if they do not match what you need, please contact the National Archives at Washington, DC - Textual Reference (RDT1) via email at Archives1reference@nara.gov.
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We hope this is helpful. Best of luck with your research!
"Topographers Office of Post Office" was listed on a page and I found the following information. I think that this is where I found the microfilm, but I'm not 100%.
There are films for states and I would assume all counties.
Publication Number: M-1126
Publication Title: Post Office Department Reports of Site Locations 1837-1950
Date Published: 1986