For what it's worth, this is what the BMC engineering documentation says about the Australian Cooper S grille panel assembly and the brace.
The Australian version of the front panel assembly (part #HYA3984) was released into production on 26/4/66 with pop-rivets specified to attach the brace (part #HYB1269). Presumably the UK pressed front panel was used prior to that date.
On 27/3/68 an amendment to delete the pop-rivets and add spot welds was released into production. A bit later on 15/7/68 the specification for the brace itself was amended to delete the four holes previously used for the pop-rivets.
Below is the full drawing, but I've also shown an exploded view of the drawing legend showing the various changes (change 3 is the one we are talking about here), and a view of the bottom attachment of the brace where you can see the number 3 in a circle next to the comments.
A circled number is used throughout engineering drawings to help locate the change referenced in the legend. You can also see the location of changes 4, 5, 7 and 9 in this view.
Let the debate begin on what actually happened in practice.
Attachment:
HYA3984 Panel Assembly Grille YDO6 wm.jpg
Attachment:
HYA3984 Legend wm.jpg
Attachment:
HYA3984 Centre wm.jpg