Amendment: SEC Form SCHEDULE 13D/A filed by Custom Truck One Source Inc.

The information set forth on the cover pages of this Schedule 13D is hereby incorporated by reference in this Item 5(a) and (b). The following sets forth, as of the date of this Statement, the aggregate number of shares of Common Stock and percentage of shares of Common Stock beneficially owned by each of the Reporting Persons, as well as the number of shares of Common Stock as to which each Reporting Person has the sole power to vote or to direct the vote, shared power to vote or to direct the vote, sole power to dispose or to direct the disposition of, or shared power to dispose or to direct the disposition of, as of the date hereof, based on 233,432,467 shares of Common Stock outstanding as of October 28, 2024, as disclosed in the Issuer's Quarterly Report on Form 10-Q as filed with the Securities and Exchange Commission on October 30, 2024. -ECP Cardinal Holdings, LP -- Amount beneficially owned: 2,847,446 -- Percent of Class: 1.2% -- Sole power to vote or to direct the vote: 0 -- Shared power to vote or to direct the vote: 2,847,446 -- Sole power to dispose or to direct the disposition: 0 -- Shared power to dispose or to direct the disposition: 2,847,446 -NESCO Holdings, LP -- Amount beneficially owned: 13,439,824 -- Percent of Class: 5.8% -- Sole power to vote or to direct the vote: 0 -- Shared power to vote or to direct the vote: 13,439,824 -- Sole power to dispose or to direct the disposition: 0 -- Shared power to dispose or to direct the disposition: 13,439,824 -Each of the other Reporting Persons: -- Amount beneficially owned: 16,287,270 -- Percent of Class: 7.0% -- Sole power to vote or to direct the vote: 0 -- Shared power to vote or to direct the vote: 16,287,270 -- Sole power to dispose or to direct the disposition: 0 -- Shared power to dispose or to direct the disposition: 16,287,270 ECP Cardinal Holdings, LP and NESCO Holdings, LP are the record holders of 2,847,446 and 13,439,824 shares of Common Stock, respectively. ECP ControlCo, LLC is controlled by its board of managers, which consists of Douglas Kimmelman, Peter Labbat, Tyler Reeder, Rahman D'Argenio, Raoul Hughes and Xavier Robert (together, the "ECP Managers"), all of whom collectively share the power to vote and dispose of the securities beneficially owned by ECP ControlCo, LLC. As a result of the relationships described below, each of the ECP Managers may be deemed to share beneficial ownership of the securities described herein. Each individual disclaims beneficial ownership of such securities. ECP ControlCo, LLC is the managing member of Energy Capital Partners III, LLC, which is the general partner of (i) Energy Capital Partners GP III, LP, which is the general partner of each of Energy Capital Partners III, LP, Energy Capital Partners III-A, LP, Energy Capital Partners III-B, LP, Energy Capital Partners III-C, LP, and Energy Capital Partners III-D, LP, which are the sole members of ECP Starlight Public GP, LLC, which is the general partner of ECP Starlight Guarantor (Public), LP, and (ii) Energy Capital Partners GP III Co-Investment (NESCO), LLC, which is the general partner of Energy Capital Partners III (NESCO Co-Invest), LP. ECP Starlight Guarantor (Public), LP and Energy Capital Partners III (NESCO Co-Invest), LP are the sole members of NESCO Holdings GP, LLC, which is the general partner of each of ECP Cardinal Holdings, LP and NESCO Holdings, LP. Each of the foregoing entities may be deemed to share beneficial ownership of the securities held of record by ECP Cardinal Holdings, LP and NESCO Holdings, LP.

See Item 5(a) above.

On December 19, 2024, ECP Cardinal Holdings, LP and NESCO Holdings, LP disposed of 699,306 and 3,300,694 shares of Common Stock, respectively, in a block trade at a price of $4.40 per share.

None.

Not applicable.