Sec Form 4 Filing - PLATINUM EQUITY, LLC @ Custom Truck One Source, Inc. - 2025-01-30

FORM 4 Check this box if no longer subject to Section 16. Form 4 or Form 5 obligations may continue. See Instruction 1(b).

UNITED STATES SECURITIES AND EXCHANGE COMMISSION Washington, D.C. 20549 STATEMENT OF CHANGES IN BENEFICIAL OWNERSHIP Filed pursuant to Section 16(a) of the Securities Exchange Act of 1934, Section 17(a) of the Public Utility Holding Company Act of 1935 or Section 30(h) of the Investment Company Act of 1940

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1. Name and Address of Reporting Person PLATINUM EQUITY, LLC 2. Issuer Name and Ticker or Trading Symbol Custom Truck One Source, Inc. [ CTOS] 5. Relationship of Reporting Person(s) to Issuer (Check all applicable) _____ Director X __ 10% Owner _____ Officer (give title below) _____ Other (specify below) (Last) (First) (Middle) C/O PLATINUM EQUITY ADVISORS, LLC, 360 NORTH CRESCENT DRIVE, SOUTH BUILDING 3. Date of Earliest Transaction (MM/DD/YY) 01/30/2025 (Street) BEVERLY HILLS, CA90210 4. If Amendment, Date Original Filed (MM/DD/YY) 6. Individual or Joint/Group Filing (Check Applicable Line) _____ Form filed by One Reporting Person __ X __ Form filed by More than One Reporting Person (City) (State) (Zip)

Table I - Non-Derivative Securities Acquired, Disposed of, or Beneficially Owned 1.Title of Security (Instr. 3) 2. Transaction Date (MM/DD/YY) 2A. Deemed Execution Date, if any (MM/DD/YY) 3. Transaction Code (Instr. 8) 4. Securities Acquired (A) or Disposed of (D) (Instr. 3, 4 and 5) 5. Amount of Securities Beneficially Owned Following Reported Transaction(s) (Instr. 3 and 4) 6. Ownership Form: Direct (D) or Indirect (I) (Instr. 4) 7. Nature of Indirect Beneficial Ownership (Instr. 4) Code V Amount (A) or (D) Price Common Stock 01/30/2025 P 8,143,635 A $ 4 156,743,635 I See Footnotes ( 1 ) ( 2 )

Table II - Derivative Securities Acquired, Disposed of, or Beneficially Owned ( e.g. , puts, calls, warrants, options, convertible securities) 1. Title of Derivative Security (Instr. 3) 2. Conversion or Exercise Price of Derivative Security 3. Transaction Date (MM/DD/YY) 3A. Deemed Execution Date, if any (MM/DD/YY) 4. Transaction Code (Instr. 8) 5. Number of Derivative Securities Acquired (A) or Disposed of (D) (Instr. 3, 4, and 5) 6. Date Exercisable and Expiration Date (MM/DD/YY) 7. Title and Amount of Underlying Securities (Instr. 3 and 4) 8. Price of Derivative Security (Instr. 5) 9. Number of Derivative Securities Beneficially Owned Following Reported Transaction(s) (Instr. 4) 10. Ownership Form of Derivative Security: Direct (D) or Indirect (I) (Instr. 4) 11. Nature of Indirect Beneficial Ownership (Instr. 4) Code V (A) (D) Date Exercisable Expiration Date Title Amount or Number of Shares

Reporting Owners Reporting Owner Name / Address Relationships Director 10% Owner Officer Other PLATINUM EQUITY, LLC C/O PLATINUM EQUITY ADVISORS, LLC 360 NORTH CRESCENT DRIVE, SOUTH BUILDING BEVERLY HILLS, CA90210 X Gores Tom C/O PLATINUM EQUITY ADVISORS, LLC 360 NORTH CRESCENT DRIVE, SOUTH BUILDING BEVERLY HILLS, CA90210 X Signatures Platinum Equity, LLC, By: /s/ Mary Ann Sigler, Executive Vice President, Chief Financial Officer and Treasurer 02/03/2025 Signature of Reporting Person Date Tom Gores, By: /s/ Mary Ann Sigler, Attorney-in-Fact 02/03/2025 Signature of Reporting Person Date

Explanation of Responses:

( 1 )PE One Source Holdings, LLC ("PE One Source") is the record holder of the securities reported herein. Tom Gores is the manager of Platinum Equity, LLC, which is the sole member of Platinum Equity Investment Holdings, LLC, which is the sole member of Platinum Equity Investment Holdings IC (Cayman), LLC, which is the general partner of Platinum Equity InvestCo, L.P., which is the sole member of Platinum Equity Investment Holdings V, LLC, which is the sole member of Platinum Equity Partners V, LLC, which is the general partner of Platinum Equity Partners V, L.P., which is the general partner of Platinum Equity Capital Partners V, L.P., which holds a majority of the outstanding equity of PE One Source. ( 2 )Additionally, Platinum Equity, LLC is also the sole member of Platinum Equity Investment Holdings V Manager, LLC, which is the sole manager of Platinum Equity Investment Holdings V, LLC. Platinum Equity, LLC and Tom Gores together hold a controlling interest in Platinum InvestCo (Cayman), LLC, which holds a controlling interest in Platinum Equity InvestCo, L.P. By virtue of these relationships, each of these entities and Mr. Gores may be deemed to share beneficial ownership of the securities held of record by PE One Source. Each of these entities and Mr. Gores disclaim any such beneficial ownership except to the extent of their pecuniary interests therein.

Remarks: Due to limitations of the electronic filing system, each of PE One Source Holdings, LLC, Platinum Equity Capital Partners V, L.P., Platinum Equity Partners V, L.P., Platinum Equity Partners V, LLC, Platinum Equity Investment Holdings V, LLC, Platinum Equity Investment Holdings V Manager, LLC, Platinum Equity InvestCo, L.P., Platinum InvestCo (Cayman), LLC, Platinum Equity Investment Holdings IC (Cayman), LLC and Platinum Equity Investment Holdings, LLC are filing a separate Form 4.

Reminder: Report on a separate line for each class of securities beneficially owned directly or indirectly.

* If the form is filed by more than one reporting person, see Instruction 4(b)(v).

** Intentional misstatements or omissions of facts constitute Federal Criminal Violations. See 18 U.S.C. 1001 and 15 U.S.C. 78ff(a).

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