I think they were building the models a LOT longer than you think.
I was wondering how long. I'll repost the December 1975 schedule for the X-Wing, Y-Wing, and TIE fighter for reference and in case others don't have this saved and are trying to follow along:
I have to make some assumptions on what the notations on the chart mean. There are numbers in some bars that are circled, and I'm assuming that the number means how many models were being worked on. I'm also assuming that the models were all hero models, and that the pyros were a separate project, or at least started in January of 1976. So the timeline as I understand it:
September 1975 - ILM begins work on effects (Lucas stated that by September 1976 ILM had been working a year)
week of December 8-12 1975 - looks like armatures were finished on Monday, Grant spent 4 days working on mountings for X & Ys, Lorne was working on 2 sets of wings, while Besley (Beasly?) was completing 2 X-Wing bodies while starting 2 more, and Mallasch was working on bodies to 2 or 3 Y-Wings.
week of December 15-19 1975 - Grant put electronics in one X-Wing for 4 days while Besley put electronics in two Y-Wings, Lorne and then Steve worked on assembly and detailing two X-Wings while Mallasch, Besley and Grell assembled and detailed the Ys (one Y each?). Also during this week Lorne was working on wings for a day and some change, while John and then Besley worked on X-Wing bodies with them ready to assemble by Friday.
week of December 22-24 1975 - Joe & Steve doing paint & finish, while another day on Y assembly before 2 days of paint & finish by Mallasch. Now we know they painted Blue Leader, took pictures or test shots of it and discovered the blue turned black with blue screen, and since it was on a plan on Friday, it had to be during these three days that they shot it.
December 25 1975 - off for Christmas Day
Friday December 26 1975 - one X-Wing (Blue Leader) and one Y-Wing (Red Jammer) get on a plane for Britain as carry on luggage.
week of December 29-31 1975 - this week has a block for paint & finish but no one assigned, but the week ends with 2 models each camera ready. I'm not sure if this is two models in addition to the one shipped off to the UK, but assuming it is and they were all heros, that would be Red Leader, Red 5, TIE Killer (Gold Leader) and blue/gray (Gold 2). Maybe these days were set aside in case they ran behind to give them some extra time to make the New Years Eve deadline.
January 1 1976 - off for New Years Day
unknown date 1976 - pyro mould created, Red 3 and Red 4 pyros first made (assumes that is why they are not in the paint area but are on the table of models)
unknown date 1976? - six models photographed on stands in a corner of the shop that looks like a paint area; Red Leader hero (appears finished), Red Leader pyro (pre weathering), Red 12 pyro (pre detailing), Red 10 pyro (barely visible), MOM Y (appears finished), triangles Y (appears finished).
unknown date 1976? - reference pictures of "Biggs Blue 3", "John D Blue 4", and "Blue 12" shot. (assumes they were all done at the same time when Red 12 was finished)
unknown date 1976? - Models are all lined up on a table for video and pictures with the model makers.
unknown date - Red 3 hero, blue/gray Y, and Red 6 pyro completed. Blue Leader returned and painted as Red 2 hero. (may have taken place at different times)
September 1976 - George Lucas says he has little ILM footage he can use at this point, asks for more money to complete film and gets half what he asked for.
weekend 9-10 April 1977 - Easter weekend break-in at ILM, at least one model stolen maybe more.
30 April 1977 - early screening of Star Wars
25 May 1977 - premiere
That's my rough timeline. I realize that the ILM schedule was just a plan, and like all plans it probably didn't survive first contact with the enemy, so it's doubtful the models were done on time. This is the only schedule I've seen from ILM, I don't know if anyone has a January or February one. From this it looks like most of the heros were finished by the end of December 1975 at early January 1976 at the latest. I know they had a lot of TIE fighters, Millennium Falcons, Star Destroyer, Blockade Runner and a bi-zillion Death Star panels to make as well. Assuming they had already blown up the 6 nice pyro models by September when Lucas was disappointed, they must have been built sometime from January to August 1976.
Since we have the Famous Spaceships article saying 5 X-heros, and the picture of Red Leader hero along with 3 partially finished heros, there must have been a fifth hero. My theory was the 5th hero was the stolen model, that was swiped before it could be photographed during the break-in which explains the lack of pics. But there's over a year between when it was probably finished and the break-in is so close to the finished film, that I really doubt that explanation.
I do love a good puzzle. Any thoughts?