It's my understanding that the front and rear giga-castings that will be used for all MYs from Austin are designed to use the structural battery pack. However, the current design (i.e., from Fremont) does not work with the structural battery packs. As I recall Elon or some other Tesla exec. has said (probably in Oct.) they have a work around for the Austin MYs in the event the 4680 batteries aren't ready for when production begins. I wonder if the Austin gigafactory could produce enough giga-castings both for their own needs as well as for Fremont for an interim/transition period while Fremont is upgrading their own factory to produce their own giga-castings. In any case the Fremont assembly line would need changes to use the giga-castings, so there would be some down time needed to retool/retrain. If this is possible then perhaps for an interim period Austin could be supplying the castings for both assembly plants while the Fremont Kato Rd. battery factory might be able to supply the 4680 cells and structural battery packs to both assembly plants. Of course, this is just speculation for how Fremont and Austin might be able to produce more similar versions of the MYs during the transition period.
FACTS
Both Fremont and Austin have been producing FRONT, as well as REAR, Megacasts for months.
Evidence in Fremont is the thousands of castings stacked on the lot in the vicinity of the GigaPress machines. See YT Fremont drone overflight videos.
They've been rolling the Rear castings inside for use. The Front castings have been stacked outside.
This is also true in Austin. Rear castings have been being made since August; Fronts started in October.
Evidence for this is available from the Joe Tegtmeyer videos, to which I'm a subscriber.
Castings in Austin are stored inside the cavernous building (next to steel stampings of doors, hoods, etc in production frames).
OPINION
All available evidence points to Battery Pack delays resulting in the build-up of parts, and Tesla's reluctance to use their 'backup plan' of 2170 packs with castings.
The most compelling evidence is indirect - there being no word of BATTERY PACK TESTING STATUS.
It's the packs, ALONG WITH the batteries, that count most. The integrity of the v2.0 chassis relies on the pack as a structural element.
All that said, Tesla would have proceeded with 2710 packs if they thought 4680 packs were too far from production-ready.
Why? Because Tesla stands to gain tremendously from the production time + cost benefits of using the 4680 structural packs.
IMHO, Tesla does not want to further complicate their logistics (supplier, shipping, delivery, service) by building different versions at different factories.
I use as evidence the deliveries this last 10 days of dashboards, interior, and other parts to Austin from Fremont.
What a mess if they start using different / different.
I speculate that Fremont will be doing a brief shutdown of the MY line in January to re-fit for MY v2.0 chassis production. Might already be happening.
Evidence? Final construction of an all-weather causeway between the MY chassis fab building and the newly completed MY Giga 4.5 Sprung Structure (tent).
Efficiency improvements all around.
Further evidence? Tesla's Fremont Logistics Lot has bunches of MS and MX going out, and less than usual EoQ MY.
100K vehicles are worth 2X 50K vehicles on the balance sheet. Good time to convert the MY lines.