Not Alkmaar but the castle tower of Middelburg. This Middelburg town mark combined with year letter M, used 1779-1781. The maker's mark represents a lock-plate, so for an unidentified silversmith, active in Middelburg around 1765-1790.
See Middelburg viewtopic.php?p=82869#p82869
Peter.
Source; J. de Bree Zeeuws Zilver, Schiedam 1978, page 152 # 628 lock-plate