When a component of the Nikkei 225 is delisted or removed, Nikkei Inc. generally replaces it with a stock from the same sector that has high liquidity. Sony Financial is a member of the “Financials” sector, which includes banks, securities firms, insurance companies and those categorized as “other financial services.” That final category also includes Japan Exchange, which is a component of the Nikkei 300.

Shares in the bourse operator jumped as much as 7.7% on Wednesday to a record high. It was also the pick of Daiwa Securities analyst Junichi Hashimoto in a note dated Tuesday, while analysts led by Keiichi Ito from SMBC Nikko Securities Co. raised SBI Holdings Inc. as the most liquid financial sector stock not included in the index, followed by Monex Group Inc. and consumer lender Aiful Corp and Japan Post Bank Co.

Changes to the Nikkei 225 frequently lead to wild moves as funds tracking the index scramble to match the new weightings. The replacement rules are just specific enough to lead to guesses as to what companies might be chosen, but vague enough to leave room for surprises.

In July, shares in Bandai Namco Holdings Inc. surged after it was a surprise choice to replace Chiyoda Corp. after the latter was demoted to the Tokyo Stock Exchange’s second section. The widely expected choice, DMG Mori Co., plunged.

The tender offer for Sony Financial runs until July 13. Nikkei Inc. is currently considering a rule change that would allow the replacement to happen when tender offer is successfully completed, instead of waiting until the stock is designated for delisting later in the month.

Shin Tamura, a senior analyst at Bloomberg Intelligence, sounded caution on Japan Post Bank. While it has a large market capitalization, he says, it’s a special case with a small free float, and it’s difficult to see it actually being selected.

SMBC Nikko also raised another possibility – that the replacement could be decided as part of the index’s annual review in September. That would allow for a sector rebalancing, putting in play names from the underweighted consumer goods sector, such as Zozo Inc., Nintendo Co., and Kenedix Inc.