4 Mon 'Bunkyūeihō' Bosen, 21 waves

The Bunkyū Eiho 4-mon was authorized in 1863 as part of the Tokugawa shogunate's attempt to address a chronic copper shortage that had plagued the monetary system for decades. By increasing the nominal value of the coin while reducing its copper content relative to earlier 4-mon types, the mint effectively debased the currency — a move that generated immediate public suspicion and resistance in daily commerce. The "21 waves" variety distinguishes itself from the more common 11-waves reverse, a detail significant enough to Japanese collectors that the two are cataloged as separate types entirely.