I believe these are supposed to be Y-wings in their original configuration. Not prototypes.
One factoid that's been around since the 70's, was that the rebels used a lot of old ships, and modify the heck out of them to keep them running. The streamlined fuselage was removed for ease of maintenance.
This is one reason a lot of us old coots thought we would be seeing shiny new X-Wings in the prequels. ;)