@ScriptingJapan For ~1600, see e.g. the letters treated by Kawaguchi sensei in her 2007 article 慶長九年九月二十七日付ローマ字書簡の日本語表記. The repository appears to be down at the　moment (but maybe it's accessible from Japan?), so here's the CiNii link instead: https://t.co/oMdOStnsTu