CAP1 / AP1529 1945 Edition:
QDH - What is causing the present interference? The present interference is caused by....
QFH - May I descend below the Clouds? You may descend below the Clouds
QGH - May I land using the procedure of Descent through Cloud? You may land using the procedure of Descent through Cloud
I suggest that QDH is a misprint for QDM.