I have written an article offering a detailed analysis of one shrine’s koema 小絵馬, small wooden prayer tablets. The article additionally provides a history of the research on koema that have focused not on illustrated prayers (as was traditional), but rather on written prayers, which is the form that predominates today. The article can be found in the Journal of Human Informatics which is published annually by The Institute for Research in Human Informatics at Tohoku Gakuin University. PDF link
Article: “A Comprehensive Survey of Small Votive Prayer Tablets” Journal of Human Informatics (人間情報学研究), Vol. 24, 2019, 15-34.