[Amended] Material event report (treasury share acquisition)

What happened

Material event report (treasury share acquisition decision) 6.3 Sejin TS Co., Ltd. Amendment report Date: August 27, 2026 1. Disclosure document being amended: material event report (treasury share acquisition decision) 2. Original filing date of the document being amended: August 25, 2026 3. Amendment details Item "1. Shares to be acquired" through "11. Other material matters relevant to investment decisions" - reason: 1. The company disclosed a material event report (treasury share acquisition decision) on July 31, 2026 and began acquiring treasury shares from August 3, 2026. It then filed a treasury-share acquisition results report on August 25, 2026, but it was subsequently confirmed that, because brokerage purchase fees were included within the acquisition amount during the on-market direct-acquisition process, part of the originally planned acquisition amount was not actually used to acquire treasury shares. 2. Accordingly, the company will carry out additional acquisition of the unacquired portion and amend the acquisition results report to match the actual acquisition, and is amending the original acquisition-decision disclosure in connection with this to keep the original disclosure and the actual acquisition consistent and to tidy up the related disclosures. 3. This amendment does not represent a withdrawal or suspension of the company's purpose of acquiring treasury shares to stabilize its share price and enhance shareholder value; it solely tidies up the existing disclosure in connection with the unacquired portion caused by the brokerage-fee inclusion and the resulting amendment of the results report. 4. After completing the additional acquisition and amending the results report, the company will convene a board meeting as promptly as possible to resolve and disclose a new treasury-share acquisition decision, with the board meeting to be held no later than September 2, 2026. 5. Following the new board resolution, the company will disclose the new acquisition decision and proceed with the acquisition, continuing its effort to stabilize its share price and enhance shareholder value. Before: all entries from "1. Shares to be acquired" through "11. Other material matters relevant to investment decisions" After: all entries from "1. Shares to be acquired" through "11. Other material matters relevant to investment decisions" (reset, as shown below)

Material event report / Korea Exchange disclosure obligation To: Financial Services Commission / Korea Exchange Date: August 27, 2026 Company name: Sejin TS Co., Ltd. Representative director: Kim In-sik Head office: 51-43 Jimun-ro, Wongok-myeon, Anseong-si, Gyeonggi-do (Tel) 031-650-3700 (Website) http://www.sejints.co.kr Preparer: (Title) Director (Name) Park Yong-seon (Tel) 031-650-3700

Treasury share acquisition decision

1. Shares to be acquired: common shares -; other-class shares - 2. Expected acquisition amount (KRW): common shares -; other-class shares - 3. Expected acquisition period: start -, end - 4. Expected holding period: start -, end - 5. Purpose of acquisition: - 6. Method of acquisition: - 7. Entrusted investment broker: - 8. Treasury shares held before acquisition: acquired within distributable profit - common shares -, ratio -; other-class -, ratio -; other acquisition - common shares -, ratio -; other-class -, ratio - 9. Acquisition decision date: - (independent/outside directors present: -, absent: -; audit committee attendance: -) 10. Daily buy-order quantity limit: common shares -; other-class - 11. Other material matters relevant to investment decisions: not applicable

[Treasury-share acquisition amount limit] 1. Statutory distributable-profit limit as of the end of the prior fiscal year: (a) net assets -; (b) capital -; (c) accumulated capital and earnings reserves through the prior fiscal year -; (d) unrealized gains prescribed by presidential decree -; subtotal (a-b-c-d) - 2. Treasury-share acquisition amount since the end of the prior fiscal year: - 3. Dividend amount and related earnings reserve resolved at the annual general meeting since the end of the prior fiscal year: - 4. Quarterly/interim dividend amount and related earnings reserve resolved by the board since the end of the prior fiscal year: - 5. Trust-contract amount: - 6. Acquisition cost of treasury shares disposed since the end of the prior fiscal year (moving-average method): - Treasury-share acquisition amount limit (1-2-3-4-5+6): -

[Treasury shares held before the acquisition decision] (unit: shares) All categories (direct acquisition on-market; direct acquisition off-market; tender offer; subtotal (a); trust-contract acquisition held by trustee; held in kind; subtotal (b); other acquisition (c); total (a+b+c)): common shares and other-class shares all opening balance -, acquired(+) -, disposed(-) -, retired(-) -, closing balance -