These are from various unofficial sources. As always with utility radio, it's only as good as the people who've taken the time to research it.
JIA## Aero, includes Tokyo Volmet, now Kagoshima (NTT closed Nazaki)
JJx This block is mostly governmental:
JJC Newspaper facsimile (Kyodo)
JJD Radio astronomy data (Ursigrams, etc), was 10415 kHz
JJD2 Radio astronomy data, was 15950 kHz
JJF2 Navy submarine comm on VLF
JJY Frequency Standards Bureau, off HF
JKx A series of JKx2n calls for NHK HF broadcasting
JMx Mostly Japanese Meteorological Agency (JMA) for wx:
JMB Call was used for wx at Nazaki, fate unknown
JMB37 JMA, Fussa, was 14665 kHz
JMC Marine Weather Agency, may be VHF only
JMG (JMG-JMG6) JMA, Intl. Regions, was Nazaki (NTT closed site)
JMH (JMH2,3,4,5,6) JMA FAX, tx now Kagoshima
JMI (JMI-JMI4) JMA, Intl. Hemisphere, Tokyo was 13963 kHz
JMJ (JMJ-JMJ6) JMA, Ibaraki Prefecture, was 18441.2 kHz (off air)
JMN22 JMA, Naha, 14940 kHz
JNx This block is for the Coast Guard:
JNA CG, Tokyo
JNB CG, Naha
JNC CG, Maizuru
JND CG, Akita
JNE CG, Hiroshima
JNG CG, Ishigaki
JNH CG, Tanabe
JNI CG, Hakodate
JNJ Coast Guard 10th Maritime Safety HQ, Kagoshima
JNK CG, Sasebo
JNL CG, Otaru
JNN CG, Shiogama
JNO CG, Kochi
JNP CG, Esashi
JNR Coast Guard 7th Maritime Safety HQ, Kitakyushu
JNS CG, Urakawa
JNT CG, Nagoya
JNU CG, Muroran
JNV CG, Niigata
JNW CG, Wakkanai
JNX CG, Kushiro
JNZ CG, Fushiki
