Colin Cantwell: "Most of these photos were taken in my backyard in 1975 after kitbashing the pre-production models for the the X-Wing, TIE, and others. Kitbashing is a method of model building that myself and a few others pioneered in the early 70's."
Cantwell: "Essentially, you take pieces from cannibalized model kits (cars, planes, trains, etc...) as well as shapes from around the office and house. Look closely and you will see pill bottle engines and Easter egg fuel tanks".
This drawing work then feeds into the work of ILM's model makers.
So, for A New Hope, there was kitbashing at the very beginning of preproduction (Cantwell), and then again during post-production (ILM).
In that picture it looks like they're greebling the Falcon, and behind them the kits are armoured vehicles, racing cars and engines (instead of aircraft, for example). I wonder if that's a way they forced a particular aesthetic?