The whole issue is bloody confusing if you ask me. Everyone has an opinion mostly different to the next person. What irritates me is there is no consistancy in the opinions from those who should know. Hell even the RACV has contridictory advice to its members on the subject.
What I do know is that there has been a change in very recent times and that change seems to be spreading.
The Department of Infrastructure and Regional Development (DIRD) have published the Vehicle Regulations which are part of the ADRs. You can read it here: https://infrastructure.gov.au/roads/vehicle_regulation/bulletin/vsb1/vsb_01_b.aspx From what I can tell its fairly straight forward. Then, as recently as february this year, the Queensland Department of Transport and Main roads published an updated Safe Towing Guide which now mirrors the DIRD's standards. They say what you must do not what you should do.
As far as shackles are concerned, they are not even mentioned in the Vehicle Regulations. They just talk about the chains and the attachment points.
The towing guide says: The safety chains must be properly connected to the tow bar with attachments capable of withstanding the specified breaking load of each chain. Do not use padlocks.
It goes on to say: Safety chain(s) must be suitably and appropriately connected to a tow bar. The use of a shackle is permitted, provided the shackle used is fit for the purpose and compatible with the safety chain in terms of strength and size.
It is the responsibility of the vehicle owner/operator to ensure that a shackle, if used to attach safety chain to tow bar, is fit for the purpose and compatible with the safety chain.
and then further on: Alternatively, the use of a shackle that is compatible with the safety chain AND is of a reputable brand. In this case, the shackle will have appropriate markings to show the brand and/or part identification sufficient to trace its brand and strength.
It should be recognised that a shackle that does not have any markings makes it difficult to establish its strength or compatibility with the safety chain. It is the responsibility of the vehicle owner/operator to satisfactorily address inquiries raised during any roadside audit by an enforcement officer.
Notes:
Usually diameter of the shackle pin is larger than the shackle body.
The strength of a shackle will vary according to its grade. An “S” grade shackle will be stronger than an “M” grade shackle of the same size.
Stainless steel shackles are generally unsuitable for trailer use due to the material’s general low resistance to bending stresses.
A Bow shackle will provide flexibility for greater angular movement compared to a “D” shackle.
From all of that, I'd say D or bow shackles...doesn't matter as long as they are rated.
