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).
Don't think kitbashing is dead.
ILM bought large quantities of new model kits and scanned the pieces. When building any new ship, the modelling team starts with scanned pieces of actual kits!
John Knoll: "Then we started to ask, โwell, what are the right pieces? What are the right kits to pull from to give us the best bang of our buck?โ The next step was actually sourcing these model kits. We set aside a budget and just bought a bunch of them on eBay; a lot of old vintage stuff."