Feb 23 (Reuters) - Veteran dealmaker Michael Klein and his partners have turned their $43 million personal investment in the blank-check acquisition firm they are merging with luxury electric vehicle startup Lucid Motors Inc into a nearly $3.3 billion paper gain, according to Reuters calculations. The gain, within just a few weeks, came from the meteoric rise in the shares of Klein’s special purpose acquisition company, SPAC Churchill Capital IV Corp. It is by far the most striking example of a Wall Street insider benefiting from the amateur trading frenzy, in shares such as GameStop Corp , sweeping the world of SPACs. Many mom-and-pop investors bought Churchill Capital IV shares hoping for a quick gain, pushing Lucid’s implied valuation from $11.75 billion at its nominal deal price to $64 billion as of Tuesday afternoon. But while the most lucky investors might have scored a gain of a few fold, Klein and his partners are sitting on a paper gain of close to 9,000%, based on shares in the SPAC they are entitled to. They are restricted from cashing out for up to 18 months, and the stock could drop considerably within that period, especially if Lucid, which has yet to sell a single car, fails to meet its ambitious production targets. A representative for Churchill Capital IV declined to comment on Tuesday. SPAC sponsors are often awarded large stakes in companies they help take public, a form of compensation known on Wall Street as the “promote.” But the gains in Klein’s SPAC shares have made his potential payout unprecedented. Klein and his partners received 42.85 million warrants in the SPAC which are exercisable at $1 apiece, according to regulatory filings. They were also given 51.75 million so-called founders’ shares in the SPAC. Churchill Capital IV’s stock closed at $35, valuing the founders’ shares at $1.8 billion. The shares available for purchase through the warrants would also be worth around $1.5 billion. It is a home run for Klein in his second act on Wall Street. He had worked at Citigroup Inc for 23 years and ran the bank’s institutional clients business, where he oversaw its advisory and financing practice. Klein was once considered a potential successor to former Citigroup Chief Executive Sandy Weill, but left in 2008. He later set up a New York-based boutique advisory firm, M. Klein & Co, and branched out into SPAC investing in 2018. The Lucid deal helps Klein regain some luster with investors who backed his previous SPAC deal for U.S. healthcare services firm MultiPlan Corp, which is trading down more than 20% since the deal closed. Shares in his first SPAC deal, a 2019 merger with analytics firm Clarivate Plc, have more than doubled since closing. (Reporting by Joshua Franklin in Miami; Editing by Richard Chang)