L79 SBC 0riginal 327 valve covers

[opelitis1]

Good morning gentlemen!!
I have 2 sets of the above valve covers with an  "0"   punched horizontally on the lower edge of the covers.  One set is definitely heavier than the other.
Can someone give me a bit of history on these covers?? What did the " 0 " signify??
One Googles and so much new non-vintage GM  stuff comes up with the response(s). 
Thanks in advance.
Ted