As far as I can see, these are the differences between the initial “Pirate Ship” model, and the final Rebel Blockade Runner used in Star Wars. It’s hard to know for certain what’s going on, since there aren’t a lot of photos available of the original Pirate Ship, and none are particularly high resolution. But these are my best guesses.
- The Pirate Ship’s cockpit was transferred to the five foot Falcon. This means that the secondary cone behind the cockpit had semi-circular notches on the Blockade Runner, since the Blockade Runner has a perpendicular double-cone hammerhead, versus the Pirate Ship’s parallel truncated cone. The frontmost cockpit cone on the Pirate Ship also appears to be slightly larger in diameter than the reverse cone behind it. That, and perhaps there’s a narrow recessed cylindrical ring between them - it’s unclear.
- The radar dish from the Pirate Ship was transferred to the five foot Falcon, and the Blockade Runner gained a replacement rectangular dish.
- The Pirate Ship’s round dish had a projecting spike antenna in the middle that was removed when the dish was transferred to the five foot Falcon. This spike looks like it was made from an Airfix Apollo escape tower, with an additional conical part on top.
- The Pirate Ship's upper spine or ridge has far fewer greeblies along its surface. A bunch of new greeblies were added when it became the Blockade Runner.
- The four topside escape pods on the Pirate Ship each have three thruster bells sticking out. This escape pod design, incidentally, more closely matches the full-sized escape pod prop used in Tunisia for the R2/3PO landing scene. Though it’s only shown off in the distance, so you can’t really tell. The Blockade Runner has single cannons on the tip of these four projecting truncated cones/domes, and the bells were removed.
- The starboard frontmost triangle panel on the Pirate Ship seems to have a projecting antenna or something. The portside one doesn’t.
- Three landing gear legs were made for the Pirate Ship, protruding from hatches.
- Interestingly the Pirate Ship had a human-height port or door on the starboard side, from which a boarding ramp could emerge. The Blockade Runner retains this port, though it doesn’t make much sense since the ship was rescaled compared to the Pirate Ship. Maybe it could be some sort of cargo port in the revised ship design. Still, the starboard side of the model was never shown in ANH anyway.