1959 to 1963
The information is right on the first page with all the credits and copyright info. I can not recall what they call that page, but it's there
In a normal book this is a title page. Here it's part of the indicia.
Showcase Presents Superman Family #1 (Out this March) will feature:
Jimmy Olsen #1-22 & Showcase #9 (1st solo Lois Lane) with contents chronologically from: Sept-Oct 1954 thru Aug 1957
Showcase Present Superman #2 (June according to Amazon's list) will probably feature: Superman # 134-145, Annual #1-2, Action #258-275
Nov 1959-April 1961