Issues until around 1959 included issue numbers (possibly at the time Amalgamated Press was sold to the Mirror Group/Fleetway Publications). After that time, only dates were used to identify the issues.
Once number stops we use year, month , date format to identify issues. For example 591205 would be issue published Dec. 5th 1959.
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.