Since I'm posting here now, Transitional 'M23' Adrian - probably used by the Spanish during their civil war (elaboration in captions) : r/Militariacollecting
Four Bees: Danish M-23 Helmet
WW2 Pre German Occupation Danish M23/39 Civil Defense Helmet And Liner
Military equipment of the 20th century: June 2009
Rare mint Danish EM M23/38 helmet dated January 1940 – fjm44