I have the first issue "Hi-Fi Stereo" labels, which I found in a manually amended (Biro) mono catalogue numbered sleeve...
...I have come across this a couple of times with these early Philips stereos now, so would appear to be a feature of early stereo Philips isssues: Stereo disc in a mono sleeve.
So, tip for the day: If you are looking for these early Plum label stereo issues, don't go looking for stereo sleeves, you might be going straight past them when you simply dismiss an apparently early mono issue!
Always check what's inside!
(Could be these plum stereos are not as rare as you might imagine.... the've just been hiding in mono sleeves... in plain sight!)
Although... the incidents when I've come across this, the tell tale biro or felt applied "S" append-ment to catalogue number on the back of the sleeve is a good indicator of plummy goodness within :)