As flagged last week, National Australia Bank announced its CPS II perpetual converting preference share issue yesterday. NAB is looking to raise at least A$750 million but did not specify a maximum amount, although market talk has set the maximum at A$1.0 billion.Market talk had also put the credit spread over the 90-day bank bill rate, used to calculate dividend payments, at 325 basis points. As it was, the indicative margin has been set at 325 to 340 bps.The book build is scheduled for next Tuesday, and the size of the issue and the credit spread to be paid will be announced on Wednesday. However, all of this may happen sooner than scheduled. A trend seems to be developing in this respect.NAB's CPS II is the fourth issue of fully compliant Basel III additional tier-one capital to be undertaken this year.Westpac sold almost $1.4 billion of capital notes in March, and NAB raised $1.5 billion from its first CPS issue in the same month. ANZ raised just over $1.2 billion from the sale of capital notes in August.Not surprisingly, the terms and conditions attached to all these issues have become fairly standard. All are perpetual securities that pay non-cumulative, fully franked dividends (if sufficient franking credits are available).However, the ANZ capital notes pay only semi-annual dividends, while the others are quarterly.All the hybrid securities are also subject to APRA's imposition of a common equity trigger and a non-viability trigger. Activation of either trigger could result in the hybrids being converted into the ordinary shares of the issuer at a value that is less than the face value of the hybrids, or the hybrids being written off entirely, if conversion is not possible.Differences arise between the issues in the areas of optional redemption or conversion of the hybrids, and in the timing of this event and the subsequent mandatory conversion event. In the case of Westpac's capital notes and NAB's CPS, the timing of the optional redemption date is six years after the date of issue, and the mandatory conversion date comes eight years after the issue date. In the normal expected course of events, investors should get their money back after six years.However, investors in ANZ's capital notes will have to wait eight years before the optional redemption date is reached, and the mandatory conversion date is another two years after that. Investors in NAB's CPS II will have to wait a year longer in each case than those holding NAB's CPS - in other words, seven and nine years, respectively.But there is a further twist.ANZ has the option of an early conversion of its capital notes into ordinary shares, rather than redeeming the hybrids. NAB also has this option with the CPS and the CPS II. This is a twist on the traditional convertible note structure, where it is the note-holder who has the option to convert into ordinary shares. Under the terms of the ANZ capital notes and the two NAB CPS hybrids, the issuer has the option