Appointments of Postmasters through 1971 are on Ancestry.com
If you have a subscription to ancestry or know someone or a library that has a subscription go to the above link and then go to image 338 (beginning of Montgomery Co in 1900). Towns are arranged alphabetically and each is followed by the names and appointment date of postmasters for roughly 20 or 30 years. To find later dates just go to the next dated section for Montgomery Co and then find Harleysville again.
Dear Mr. Bucher,
Thank you for posting your request on History Hub!
We searched the National Archives catalog and located the Records of Appointments of Postmasters, 1832 - 9/30/1971 in the Records of the Post Office Department (Record Group 28) that should include the postmasters for Harleyville in Montgomery County, PA from 1900-1970. We also located the Records of Appointment of Postmasters and the Establishment of Post Offices, 7/1/1931 - 10/31/1971 in Record Group 28 that includes two digitized file units for Montgomery County, Pennsylvania. For access to the non-digitized records, please contact the National Archives at Washington, DC - Textual Reference (RDT1) via email at email@example.com.
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We hope this is helpful. Best of luck with your research!