Some history / info here:
http://www.ejectionsite.com/escapacfr.htm
"...The Skyhawk used several different Escapacs over the years and each of the seats was an improved version of the prior version. Some other changes involved the catapult and rocket, the parachute, as well as canopy breakers and jettison systems...."
"Designation - Aircraft
1 - A-4A, B, C,
IC-3 - A-4F, TA-4F, and SUBS
IF-3 - A-4F & TA-4
IG-3 - A-4F, TA-4F & SUBS