I
just went through this with my 105 drivetrain upgrade.
The RD-5700-A has different derailleur hanger arm parts than the RD-5700. Parts 8 and 10
here.
The RD-5701 got new inner and outer plates again, and the GS version got it's own separate B-Axle assembly (the housing with "105" printed on it and associated parts) than the SS version. Parts 8, 10, and 1
here.
Long story short, the 5701 replaces the 5700-A, which replaced the 5700. In any case, when your LBS orders your new 105 RD they'll probably only be able to get the 5701. That was my experience.
Look at the links to the exploded view diagrams above. The GS hanger arm is noticeably longer, like an MTB RD.
No, there is no part number RD-5703.
Short cage is SS (28t max), mid cage is GS (30t max, but 32t may work depending on the derailleur hanger). Black has an -L suffix (with silver hanger), Silver has an -S suffix (with black hanger).
