There are several 春日 (Kasuga) places in Japan, but two are relevant here — one as the most famous national reference, and one as the locally plausible reading for the manuscript’s place-name. Kasuga Taisha in Nara is the most famous. The historically and culturally dominant 春日 is in Nara Prefecture (former Yamato Province), in the eastern edge of the city of Nara. It refers to the area around Kasuga Taisha (春日大社), one of the most important shrines in Japan, founded in 768 CE by the Fujiwara clan. Kasuga Taisha enshrines Takemikazuchi (建御雷神 / 武甕槌神) — the same martial deity invoked in the Heihō Denki preamble. The 768 founding involved transferring Takemikazuchi from Kashima Shrine in Hitachi Province to Nara. The myth holds that the deity rode from Kashima to Kasuga on a white deer, which is why the deer of Nara Park are sacred to this day.
So, Kashima (Hitachi) and Kasuga (Nara) are the two ends of a single transferred deity. The Kashima Shrine is the origin point; the Kasuga Taisha is the Yamato terminus. For a school that anchors its origin in Kashima, Kasuga is the sister-shrine carrying the same Takemikazuchi-as-martial-deity tradition into the heartland of imperial-Fujiwara culture.
If the characters are reversed and should be 常陸國春日 — Hitachi Province, at Kasuga — then this would be referring to a local Kasuga place-name within Hitachi Province, almost certainly a satellite location near Kashima Shrine itself. Modern 鹿嶋市 (Kashima City) in Ibaraki Prefecture has small place-names that include 春日 references, and many shrines designated 春日神社 (Kasuga Jinja) exist throughout the old Hitachi Province as branch shrines of the Nara Kasuga Taisha — established in places associated with the Takemikazuchi cult or the Fujiwara/Nakatomi clan network that radiated from Kashima.
For Sugimoto’s residence to be at a Hitachi-Kasuga location near Kashima Shrine would support the densho’s narrative: it would put the school’s founder living next to the shrine where his dream-transmission occurred, in a sub-locality named for the deity’s Yamato-terminus partner-shrine — with Sugimoto positioned at the Hitachi-Kasuga end of that single Takemikazuchi system as a local adherent.
