There are essentially three different ‘G’ heads.  While all the small letter ‘G’ heads are unposted, the large letter ‘G’ heads can be unposted or posted.  Supposedly all heads cast after May 15, 1957 are posted but I have found exceptions to that.  The date code on the ‘G’ heads is located under the valve cover and more specifically under the rocker shaft on the right side of the head when viewing from the exhaust side of the engine.  Left side of the head when viewing from the intake side of the head.  The first character indicates the year, the next character(s) indicates the month, and the next character indicates the day.
 
Here are pics of a pair of posted heads and their date codes.  This pair is originally off of a 1957 Thunderbird and both look to be original to the engine.
  
 
1957 May 28
 
  
1957 June 5
 
Here’s the link to the past thread on this site about how to identify posted heads if they are off the engine.
http://forums.y-blocksforever.com/FindPost6764.aspx

Lorena, Texas (South of Waco)

Again thank you, both Noshortcuts and Ted. Guess I'll have to clean some sludge to find the date code, but will also check out what I can see/feel through the holes in the deck.

After looking at these heads last night they aren't quite as bad as I remembered. But, one head has what looks to be evidence of a swallowed bolt. Right on the edge of the combustion chamber and the quench area there is what looks like threads rolled into the cast iron. Not sure how well this will clean up, even with a full cut. Guess I'll find out. Everybody have a great Y-Block day. -Dave.