Issues until around 1959 included issue numbers (possibly at the time Amalgamated Pess was sold to the Mirror Group/Fleetway Publications). After that time, only dates were used to identify the issues.
Marilyn was merged into Valentine on 25 September 1965.
In the Complete Catalogue of British Comics, Denis Gifford reports 547 issues from 19 March 1955 to 18 September 1965. There are 549 weeks between those two dates, suggesting two weekly issues were not published or Gifford miscounted.