Regarding the pensionado architects, Juan Arellano, Antonio Toledo, and Tomas Mapua, one place to start is with their alma maters in the United States and contacting their archives to see if they retained any of their respective papers. Arellano completed his degree at Drexel University Drexel University Archives | Drexel Libraries , Antonio Toledo at Ohio State University University Archives - Ohio State University Libraries , and Tomas Mapua at Cornell University Cornell University Archives | Rare and Manuscript Collections
During the pensionado architects work, the Philippines were governed by the Insular Government of the Philippines. This administration operated under a agency called the Bureau of Insular Affairs which oversaw the different departments of the country, including public works, civil service, legislative offices, and the various state offices. NARA holds some documents from the Bureau (Record Group 350, Records of the Bureau of Insular Affairs) and manuscripts from the Philippine Commission from this same time period (https://catalog.archives.gov/id/657046). If you want to learn more about this collection, you can send an email to Archives2reference@nara.gov and receive more specific information about the contents of this record group.
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