Nice covers. The very feint line through the O on both is not a crack but a casting flaw and being feint would put them mid '66 as the flaw line increased in visibility as more castings were done. Those that prefer originality would like those.
From Noland Adams book:
"During the 1966 model year, the mold used to form the L79 engine's cast aluminum valve covers developed a line, which first appeared as a vertical ridge running across the O".
So your valve covers were used from mid model year 1966 through 1967.