htaccess redirect to production when assets 404

I’d seen this mentioned at a WordPress Wellington meetup a while back and couldn’t remember the syntax; luckily I found it again on this Jonathan Nicols site

# Load media files from production server if they don't exist locally
<IfModule mod_rewrite.c>
  RewriteEngine on
  RewriteBase /
  RewriteCond %{REQUEST_FILENAME} !-f
  RewriteCond %{HTTP_HOST} ^localsite\.dev$
  RewriteRule ^wp-content/uploads/(.*)$$1 [NC,L]

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