(Bloomberg) -- The Bank of Japan’s stock continued its head-scratching surge on Friday, rising for a fifth straight session.While the gains were smaller than the limit-up rallies in the previous four days, the stock has now doubled in value over the course of this week, even as the benchmark Topix index is little changed.The jump in the shares, or subscription certificates as the BOJ refers to them, has baffled market participants. While the BOJ is unusual in being a listed central bank, the stock pays a tiny dividend and holds no voting rights. In fact, the central bank doesn’t even hold shareholders’ meetings. The stock traded at an all-time low just in January.Usually little noticed or commented on, the central bank’s stock became a topic of conversation in parliament on Friday, where BOJ Governor Haruhiko Kuroda was delivering his semi-annual report on currency and monetary control. “The Bank of Japan’s subscription certificates are completely different from the normal shares of listed companies,” Kuroda said in response to a question about the long-term decline in the stock price during his term, prior to this week’s rally. Unlike a normal stock, he said, the share price doesn’t reflect profits or the state of its balance sheet. “The price is not the responsibility of the bank.”Trading volume on Thursday rose to the highest on record, according to data compiled by Bloomberg, though the 11,600 shares exchanging hands is minuscule compared to most other listed companies in Tokyo. Volume on Friday stood at around 75% of that amount after less than 90 minutes of trading.Speculative MovesWhile technically a listed entity on the Tokyo Stock Exchange’s Jasdaq, the nature of the stock and its speculative moves are a long-standing mystery of Japan’s financial markets. A speculative surge was also witnessed in late 2012 and early 2013, when optimism over the Abenomics program of former Prime Minister Shinzo Abe was at its peak.The government holds a 55% stake, while individual investors have 40%. The subscription certificates can’t be bought at online securities firms, as they weren’t subject to the 2009 digitalization of traditional paper stock certificates. It’s the only issue for which Japan Securities Clearing Corp., the entity that clears transactions for all equities in the country, still requires physical delivery of paper certificates, which remain valid even now.The puzzling rally comes after the Nikkei 225 Stock Average briefly touched its highest levels since the bubble era of the 1980s. In those days, when the benchmark traded at around 70 times earnings, some investors collected BOJ’s stock, framing their certificates as a collectible. They were then worth 745,000 yen ($6,945) apiece -- around 13 times their current value -- despite the recent surge.The gains have come ahead of a closely-watched policy review by the central bank on March 19, which may lead to changes in how it buys exchange-traded funds -- because as well as being listed, the BOJ is itself the largest single owner of Japanese shares.(Updates with Kuroda comment, Friday’s moves)For more articles like this, please visit us at bloomberg.comSubscribe now to stay ahead with the most trusted business news source.©2021 Bloomberg L.P.