airdaleman, The first versions of the model 6 did not have any gills and did not hold the bolt back. These features were added very soon after the introduction and then were kept so it appears they were done to solve problems the were found with early guns. I have not found a Savage parts catalogs that has the early parts listed, they just have a note that says early guns will be updated to the later configuration if sent to the factory.
Here is some more information -
topics/7305119/#Post7305119 
