credit for the stand in of Luke goes to John Schulze
The drawing hanging in the background was the original design for the rebel troop transports that are modeled in the foreground. What seems to have ended up happening, is that the design for the modular parts of the transport were transformed into two things. One are those boxes in the background, which appear in Yavin Base, here and there,
Then, cutting the modules in half (eliminated the "W" shape), they became the skirting for the actual transports that appear in film. More clever reuse of parts.