Good morning Wayne.
Can you locate the model identification plate for the mower and provide the details?
The best I have found, with basic information, is a Greenfield 30" deck for sale on eBay, which you may be able to identify the parts you have.
http://www.ebay.com.au/itm/GREENFIELD-30-DECK-RIDE-ON-MOWER-/201543161274?hash=item2eece895ba:g:RocAAOSw5VFWQELt
The first photo they have is an overhead view and may help identify your cutting deck. The support bracket may be part of the assembly to the left of the centre spindle. The 5th photo shows a support bracket, which appears to look like yours, that is mounted on the right hand side near the chute and goes toward the front (or back) of the deck toward the wheel, forming a triangle. The one under the piece of chain.
As there are no bolt holes on the two "Stiffeners", and looking at the both ends on the bottom photo, this could indicate they may be held in place by some other cross-mounting. Maybe have a look under the machine and see if you can spot an area that is clean where these may have been mounted?
Without the actual model identification details of the machine, this is all speculation, but may help.
