BTL-B, S3 and A4 are all variant models of the same basic Y-wings. Once you strip away the armor plating the only difference is the cockpit and turret armament. Han Solo in the ANH radio drama even says the ships are 20 years old. Sounds like the rebels got ahold of Y-wings that the empire didn't want or need anymore and converted them to single crew configuration, or the republic/empire had commissioned single seat variants and then dumped them in favor of TIEs.
So the turret, weapons, engines are completely different, the cockpit is significantly "modified", we have no idea about the internal stuff... As I said, the difference is way bigger than between T-65 and T-70.
