I'll just take a "wild stab", or a "shot in the dark", as they say.
Since the '68 #16
heads were followed by #62
heads, on similar '69 engines, and since your #16
heads were cast late in the '68 model year, I suppose it MIGHT be possible that some of the left over #16
heads were used on '69 model year engines. Also, some #62
heads may have been used on some late '68 model engines. Lots of strange things happened on the production lines, that were not common practice. So, rather than hold up production, waiting on a set of #62
heads, they may have used some left over #16
heads on a few engines. Stranger things have happened on GM production lines.
OR, the stamped #62 may have absolutely nothing to do with the cast #16, or a the year model of the engine. The guy that added the stamped numbers may be one of the few, or maybe even the only guy who knows for sure.