"Y. Mirsky in his dissertation on R' Kook notes that – Rav Kook wrote an approbation to the Liqutei Halakhot, a volume by the celebrated sage and saint Israel Meir Kagan (a.k.a. Hafetz Hayim) on the laws of Temple sacrifices, understandably, a less-visited precinct of Talmudic study.
I noticed a first-hand report (based on a conversation with R' Kook) in HaHed (7:6) that states that the Chafetz Chaim has initially asked R' Kook to help him write the sefer:"
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(Source: Ishim V'Shitot Blog )