FirstMac may be seeking superior terms from MyState Financial for its planned takeover of The Rock Building Society, with FirstMac lifting its stake in the society to 12.7 per cent, from 10.4 per cent, over the last week.Brisbane-based FirstMac is a mortgage manager with a long-standing interest in widening its product set through an alignment with a deposit-taking entity. The firm has built its stake in The Rock over four years and supported reform of the board at past annual meetings.MyState, based in Tasmania, and The Rock, based in Bundaberg, announced the merger (by way of scheme of arrangement) last week.MyState is offering 7.75 of its shares for 10 shares in The Rock, an offer that valued The Rock at A$2.71 a share when it was announced last week. Shares in The Rock have traded in a range from $2.57 to $2.80 since then, with FirstMac able to acquire most of the shares in the last week for less than $2.70.The Rock requires 75 per cent of shares to be voted in favour of the scheme at a special meeting, which is expected to be held next month.