help! AMC Head casting # decipe

["dmsrox2" <Donnie@xxxx>]

I have a set of heads that I was told came off of a 1968 390. The 
casting number on each is "3188558-2". It also has a big "X" just 
above the rear spark plug on each head.

Can anyone verify that that these are '68 390 heads? and is there 
any significance to the "X"?

Thanks in advance for your help!!

Donnie Solomon