SCHEDULE 13D/A 0001246840 XXXXXXXX LIVE 1 Common Stock 10/17/2025 false 0001709682 23204X103 Custom Truck One Source, Inc.

7701 Independence Avenue Kansas City MO 64125

Mark Ein 202-654-7001 2200 Wilson Blvd, Suite 102 #447 Arlington VA 22201 Y Capitol Acquisition Management IV LLC WC OO N DE 0.00 0.00 0.00 0.00 0.00 N 0 OO Y Capitol Acquisition Founder IV LLC WC OO N DE 0.00 0.00 0.00 0.00 0.00 N 0 OO 0001246840 N Mark Ein OO N X1 5159319.00 0.00 5159319.00 0.00 5159319.00 N 2.3 IN Y Dyson Dryden OO N X1 0.00 0.00 0.00 0.00 0.00 N 0 IN Common Stock Custom Truck One Source, Inc. 7701 Independence Avenue Kansas City MO 64125 This Amendment No. 1 to Schedule 13D (this "Amendment No. 1") amends and supplements the statement on Schedule 13D filed with the United States Securities and Exchange Commission on April 14, 2021 (the "Schedule 13D"), relating to the common stock, par value $0.0001 per share (the "Common Stock"), of Custom Truck One Source, Inc., a Delaware corporation (the "Issuer"), whose principal executive office is located at 7701 Independence Ave., Kansas City, MO 64125. Capitalized terms used herein without definition shall have the meaning set forth in the Schedule 13D. Item 4 of the Schedule 13D is hereby amended and supplemented with the following: Distribution In-Kind On October 17, 2025, Management IV distributed 3,670,279 shares of Common Stock to its members without consideration on a pro rata basis. As part of this distribution, Mr. Ein acquired 3,553,612 shares of Common Stock. Item 5 of the Schedule 13D is hereby amended and restated in its entirety as follows: The beneficial ownership set forth herein is based on 226,559,586 shares of Common Stock outstanding as of July 28, 2025, as disclosed in the Issuer's Quarterly Report on Form 10-Q filed with the Securities and Exchange Commission on July 30, 2025. Mr. Ein is the record holder of 5,139,319 shares of Common Stock and options to purchase 20,000 shares of Common Stock that are currently exercisable. Other than as described in Item 4, the Reporting Persons have not effected any other transactions in Common Stock during the past 60 days. None. The Reporting Persons have ceased to be the beneficial owners of more than five percent of the outstanding Common Stock. Capitol Acquisition Management IV LLC /s/ Mark Ein 10/21/2025 Capitol Acquisition Founder IV LLC /s/ Dyson Dryden 10/21/2025 Mark Ein /s/ Mark Ein 10/21/2025 Dyson Dryden /s/ Dyson Dryden 10/21/2025