Your bow should be a 1967 bow if it is an Explorer II. The following dates were used:
The original Explorer had no numerical designation and was 62" long. Usually a solid wood riser...
EXPLORER 62" 1963-67 (Top of line hunting/target 30# to 60#)
Later they came out with a short 56" version and hence the beginning of the I and II descriptions. The first number relates to the year produced...there are some strange ones for transition years in labeling and design and such...but this holds pretty true to form.
EXPLORER I 56" 1967-75 (Short version replaced Explorer)
EXPLORER II 62" 1967-75 (Long version replaced Explorer)
Would love to see pics of it if you still own the bow....