ATCO-G Atlas Corporation

As filed with the Securities and Exchange Commission on March 28, 2023

Registration Nos. 333-200639

333-195571

UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

Post-Effective Amendment No. 4

to Form F-3 Registration Statement No. 333-200639

Post-Effective Amendment No. 4

to Form F-3 Registration Statement No. 333-195571

UNDER

THE SECURITIES ACT OF 1933

23 Berkeley Square

London, United Kingdom

W1J 6HE

Telephone: +44 20 7788 7819

(Address, including zip code, and telephone number, including area code, of registrant’s principal executive office)

Puglisi & Associates

850 Library Avenue

Suite 204

Newark, Delaware 19711

Telephone: (302) 738-6680

(Name, address, including zip code, and telephone number, including area code, of agent for service)

Copy to:

Hillary H. Holmes

Mark D. Director

Gibson, Dunn & Crutcher LLP

811 Main Street, Suite 3000

Houston, Texas 77002

Telephone: (346) 718-6600

Approximate date of commencement of proposed sale to the public: Not Applicable.

If the only securities being registered on this Form are being offered pursuant to dividend or interest reinvestment plans, please check the following box.  ☐

If any of the securities being registered on this Form are to be offered on a delayed or continuous basis pursuant to Rule 415 under the Securities Act of 1933, check the following box.  ☐

If this Form is filed to register additional securities for an offering pursuant to Rule 462(b) under the Securities Act of 1933, please check the following box and list the Securities Act registration statement number of the earlier effective registration statement for the same offering.  ☐

If this Form is a post-effective amendment filed pursuant to Rule 462(c) under the Securities Act of 1933, check the following box and list the Securities Act registration statement number of the earlier effective registration statement for the same offering.  ☐

If this Form is a registration statement pursuant to General Instruction I.C. or a post-effective amendment thereto that shall become effective upon filing with the Commission pursuant to Rule 462(e) under the Securities Act of 1933, check the following box.  ☐

If this Form is a post-effective amendment to a registration statement filed pursuant to General Instruction I.C. filed to register additional securities or additional classes of securities pursuant to Rule 413(b) under the Securities Act of 1933, check the following box.  ☐

Indicate by check mark whether the registrant is an emerging growth company as defined in Rule 405 of the Securities Act of 1933.

Emerging growth company  ☐

If an emerging growth company that prepares its financial statements in accordance with U.S. GAAP, indicate by check mark if the registrant has elected not to use the extended transition period for complying with any new or revised financial accounting standards provided pursuant to Section 7(a)(2)(B) of the Securities Act.  ☐

EXPLANATORY NOTE

These Post-Effective Amendments No. 4 (“Post-Effective Amendments No. 4”) to the Registration Statements on Form F-3, File Nos. 333-200639 and 333-195571 (as amended, collectively, the “Registration Statements”), are being filed by Atlas Corp., a Republic of the Marshall Islands corporation (“Atlas” or the “Registrant”), as the successor registrant to Seaspan Corporation, a Republic of the Marshall Islands corporation and the former publicly-traded parent of Atlas, to deregister any and all common shares, par value $0.01 per share, of Atlas (the “Common Shares”) registered by the Registrant that remain unsold pursuant to the Registration Statements.

Effective as of March 28, 2023, as contemplated by the Agreement and Plan of Merger, dated as of October 31, 2022, by and among the Registrant, Poseidon Acquisition Corp. (“Poseidon) and Poseidon Merger Sub, Inc., a wholly-owned subsidiary of Poseidon (“Merger Sub”), Merger Sub merged with and into the Registrant with the Registrant continuing as the surviving corporation and a wholly-owned subsidiary of Poseidon (other than with respect to the Registrant’s preferred shares) (the “Merger”).

As part of the Merger, the Registrant has terminated the Registrant’s securities pursuant these Registration Statements (other than with respect to the Registrant’s preferred shares). In accordance with the undertakings made by the Registrant in these Registration Statements to remove from registration, by means of a post-effective amendment, any of the Registrant’s Common Shares that remain unsold at the termination of the offering, the Registrant hereby removes from registration, by means of these Post-Effective Amendments No. 4 to the Registration Statements, any and all Common Shares registered under these Registration Statements that remain unsold as of the effectiveness of the Merger on March 28, 2023.

SIGNATURES

Pursuant to the requirements of the Securities Act of 1933, the registrant certifies that it has reasonable grounds to believe that it meets all of the requirements for filing on Form F-3 and has duly caused these Post-Effective Amendments No. 4 to the Registration Statements to be signed on its behalf by the undersigned, thereunto duly authorized, in Hong Kong, Hong Kong on March 28, 2023.

AUTHORIZED REPRESENTATIVE

Pursuant to the requirement of the Securities Act of 1933, the undersigned, the duly undersigned representative in the United States of Atlas Corp., has signed these Post-Effective Amendments No. 4 to the Registration Statements in the City of Newark, State of Delaware, on March 28, 2023.