X-heads vs J-heads: what's the difference??

Code-Schmode,

Castings #_______Valve Size_____Code_________Year_________Engine

2531894 .............. 2.02" x 1.60" ........ 'X' ................. 1968 thru 1971 .... 340

3418915 .............. 1.88" x 1.60"......... 'J' 'Z' 'O' ........ 1970 thru 1972 .... 360

3418915 .............. 2.02" x 1.60" ........ 'O' 'U' ............ 1970................... 340 T/A

3418915 .............. 2.02" x 1.60" ........ 'J' .................. 1971 .................. 340

3418915 ............. 1.88" x 1.60" ..........'J' .................. 1972 .................. 340