The original ASDS (Autonomous Spaceport Drone Ship known as "Just Read The Instructions" (JRTI) was based on the Marmac 300 hull. It has been replaced with the Marmac 304 hull for use in the CRS-7 launch attempt of a first stage landing.
Marmac 303 has the wings from Marmac 300 and presumably will become the West Coast ASDS after a trip through the Panama Canal with the wings removed. Per Elon Musk, the West Coast ASDS will be named "Of Course I Still Love You" (OCISLY). Edit: In hindsight a few years later, OCISLY remained the East Coast/Atlantic barge, and JRTI moved over to the West Coast/Pacific.
What changes were made to the East Coast ASDS in the switch from Marmac 300 hull to the Marmac 304 hull?