IMB shareholders have sold down the building society's stock over the past month, giving the thumbs down to a buyback plan that the IMB board announced at the end of May.IMB has a hybrid structure; it is a mutual approved deposit-taking institution and also a public company limited by guarantee and shares. The shares are traded on an exempt market operated by IMB.In pursuit of a growth strategy, the board commissioned consultants Grant Samuel Corporate Finance and Watson Mangioni Lawyers to work on its structure.The consultants' view was that the ordinary shares had become an expensive form of capital for IMB because of the level of dividend payments. The existence of the shares also stood in the way of the group taking up merger opportunities with other mutuals.IMB has 39.9 million shares on issue and 4,200 shareholder members among its 180,000 members.Those shares have fallen 25 per cent since the release of the review at the end of May. The share price has fallen from A$3.74 on May 24 to $3.40 on May 31 (the day after the board's announcement) to $2.81 this week.