Re old (1992) US VFR sectional charts-- when was the thin dashed magenta line first used?
The decision to change the airspace classification in the US to bring it into agreement with ICAO classifications was made in December 1991, as can be seen in the US Library of Congress:  Federal Register: 56 Fed. Reg. 65415 (Dec. 17, 1991).

The ICAO classification was only used from the 16th of September 1993, but as part of the familiarisation process for pilots, the legends of the charts were already updated before. 

The Federal Register (FR) contains a table with the Airspace Reclassification Transition timeline.



This table shows that as of October 15, 1992, the first sectional aeronautical charts (SAC), world aeronautical charts (WAC) and terminal aeronautical charts (TAC) are published with legends that indicate both existing and future airspace classifications. 

Furthermore the FR specifies that it is anticipated that a segmented magenta line will be used for class E in the future, and that starting October 15, 1992, the new colours will be used in the legends. 


  An anticipated modification to the symbols on aeronautical charts is
  the addition of a segmented magenta line to represent the controlled
  airspace area for airports without operating control towers that
  extends upward from the surface (Class E airspace).
  
  [....]
  
  The legends in aeronautical charts
  will include both the existing airspace
  classifications and the airspace
  classifications to be effective September
  16, 1993. For example, the solid blue line
  that symbolizes a TCA will be followed
  by "TCA (Class B)." The first charts
  with a dual legend will be published
  October 15, 1992. Commencing
  September 18, 1993, the legends on these
  charts will be reversed (e.g., a solid blue
  line will be followed by "Class B
  (TCA)". Between March 3 and August
  17, 1994, the use of dual indication
  legends will be phased out.