Hey Nubee..
So close! But its actually just a bit simpler.. your meta ‘key’ only needs to be the name of the group, then underscore, then the name of the sub-field… so in your case:
$metaQuery = [
'relation' => 'AND',
[
'key' => 'stamps_local',
'compare' => '=',
'value' => true,
],
];
Hope this helps!!