what is the difference?

[beemerten]

I have found out that my firebird have a 400 engine from 1975.

I found a site with this information:
185 400 A YT 7.6 66 6X 4 76
185 400 A YT 7.6 66 5C 4 75


The only thing i can see is different is 6x/5c.
This is the heads right?
If that is right mine should have 5c but i have the 6x.
Anybody knows what the different between this two is?

 

[melsg5]

The 5C were early production and the the 6X were late production

 

[beemerten]

Thanx for the answer!