I don't recall a #2688 head - there was, however, a #2686 head.
#2686 - 1987-1990 AMC 242ci inline six (A/K/A "RENIX.") First-generation cylinder head for AMC242, noted for having the inlet and exhuast ports on a common line (they were raised ~5/8" later to improve intake breathing and airflow.)
All AMC242 cylinder heads will interchange with all blocks, but there is a dividing line at 1990/1991 for the intake manifold (due to ports being raised, as I'd just mentioned.) Any AMC242 cylinder head will also fit on a later "Wide Deck" AMC258 engine block (1975 and later, I think,) which will both improve compression and breathing. Make sure you have casting #2685 on your 258 before you try this.
Special screws or bushings may be required - the AMC258 used 7/16" cylinder head screws, while the AMC242 uses 1/2" screws.
Is there anything else you'd like to know?