Out of curiosity, how do you know this?
Here's a little information from the Model 3 order spreadsheet. This data is only for Model 3 Long Range orders in the US, and remember that the data is not even close to being exhaustive.
For the following I've defined "west coast" as CA, WA, OR, NV, AZ, ID, MT, WY, CO, and NM.
1. This is a scatter plot of dates where VINs were handed out, colored by west coast versus east coast. The date markers along the bottom indicate the start of each quarter. The vertical axis is just the delay between ordering and getting the VIN, and it is primarily used to space out the data points. The pattern seems pretty clear that there is a quiet time at the start of the quarter, then east coast VINs go out, followed by west coast VINs. Some quarters are more striking than others in adhering to the pattern.
2. This is the same style scatter plot for dates of vehicle delivery where the vertical axis again indicates the delay between order and delivery.
Note that 60% of orders on the spreadsheet are for eastern states.