THE mystery behind the sudden sharp rise in San Miguel's share price was solved yesterday when SHK Hong Kong Industries said it had agreed to sell its holding of 50.5 million shares at $7.50 each to an independent third party. The shares represent 13.5 per cent of San Miguel's issued share capital. SHK was San Miguel's second-largest shareholder behind San Miguel Corp, which holds 60.4 per cent. SHK will receive $378.75 million for its shares, accounting for 20.9 per cent of its net tangible assets as of last November 30. The value of the shares in SHK's audited financial statement at December 31, 1992 was $181.8 million. SHK's wholly-owned subsidiary Long Image bought the San Miguel shares in January 1991 from Neptunia Corp. Neptunia has the right to buy the shares back by matching the price offered by another party within five days of being notified by SHK. Neptunia, which owns 11.9 per cent of SHK, was given notice yesterday. San Miguel's stock was a lacklustre performer throughout 1993, hovering around $4 while the Hang Seng Index soared 115 per cent. However, the stock suddenly came to life on December 23 when it jumped $1.225 or 32.5 per cent to $5.80 on speculation that San Miguel might move its brewery to China and re-develop the Sham Tseng site. The company issued a statement that it knew of no reason for the jump, but the stock continued to gather momentum and hit a record of $7.60 on December 30. Daily trading volume, which had barely cracked 200,000 shares all year, ranged between two million and five million. After trading closed on December 30, managing director Stuart McGregor said the brewery would not be moved, particularly because $450 million had been spent on expansion in 1991. The stock then lost steam, falling 45 cents on Friday and a further 25 cents yesterday to close at $6.90. Trading volume plummeted to 1.05 million shares yesterday, the first day when it fell below two million in the past seven trading days.