Some relevant authorities:
"4.07 Acquired. A vehicle is not “acquired” before the date on which title to that vehicle passes under state law." IRS Notice
https://www.irs.gov/pub/irs-drop/n-09-89.pdf
You might think that the normal UCC rule on sale goods applies which would state: "(2) Unless otherwise explicitly agreed title passes to the
buyer at the time and place at which the
seller completes his performance with reference to the physical delivery of the
goods,"
§ 2-401. Passing of Title; Reservation for Security; Limited Application of This Section. That would suggest that all deliveries made in June will count in June.
But at least some states, perhaps most or all, separately regulate the passage of title of autos. For instance California courts considered this issue and a court stated:
"The transfer of title to a vehicle registered in California is accomplished under Vehicle Code section 5600: “No transfer of the title or any interest in or to a vehicle registered under this code shall pass, and any attempted transfer shall not be effective, until the parties thereto have fulfilled
either of the following requirements: (1) The transferor has made proper endorsement and delivery of the certificate of ownership to the transferee ... and the
transferee has delivered to the department ... the certificate ....”; or “(2)
The transferor has delivered to the department ... the appropriate documents for the registration or transfer of registration of the vehicle....” (Veh.Code, § 5600, subd. (a).)"
Quartz of S. California, Inc. v. Mullen Bros., 151 Cal. App. 4th 901, 907–08, 61 Cal. Rptr. 3d 54, 58 (2007) That was back in 2007 and the current law is still roughly the same.
California Code, Vehicle Code - VEH § 5600 | FindLaw
So title is not transferred until either the buyer delivers the signed over certificate of title to the DMV or the seller has delivered the title transfer paperwork to the DMV. Tesla can control both of those by not signing over the title to the buyer so the buyer could never deliver a signed over title cert (this is what would happen in a private sale), and they could sit on the paperwork until July 1 for a bunch of deliveries made in June.
If all M3s are originally titled in California the above California rule may allow Tesla to control the passage of title for all states where title is moving from one state to another. Interesting issue if states have different rules, but probably they are all similar or can at least rely on the Cal rule.
If this is right, Tesla could do a bunch of deliveries at the end of June and not send the paperwork in until July 1 and the June deliveries should count for July for purposes of IRC 30D.