[Mr70]

Rixls6
You're getting a lil backwards.Re-read the posts again,and you'll see it makes sense what the others have submitted.
X J is a Manual trans application,and X F is a TH400 application.
When any 1970 LS6 Chevelle was special ordered with the 4:10 posi upgrade,that canceled it's standard Big Block 3 core radiator,& it came instead with the larger capacity Big Block 4 core radiator.Which could have been either the PA manual prefix,consisting of the I F driver side tank,and X J Pass. side tank,or the PD TH400 prefix,which consisted of the I F driver side tank,and X F pass.side tank.
Understand that these two radiators were used as the Big Block Heavy Duty cooling radiators on the 1970 LS-6 Chevelle/Elcamino,but not exclusive to the them,as they also came as standard equipment on the 1970 L-34 & LS-5 Chevelle/Elcamino when equipped with optional Air Conditioning,or if just RPO KO-1 H.D cooling was ordered on them and that would include the 1970 L-78 Chevelle/Elcamino too.
In 1971,the PA & PD radiators were also available on all the BB Chevelle/Elcaminos under the similar circumstances mentioned above.
I believe your C I Passenger side tank was used on the 1972-74 GM A-bodies.