I've had a cable like this for many years although mine supports ACB stereo sound.
One thing to note about Spectrum +3. Almost all of them have a sound fault and have distorted sound. If you've never listened to one that has been fixed you may never even thought something was wrong.
The difference after the fix is like night and day. I would really encourage all +3 users to check their boards and see if they need to fix them. At the same time you can add stereo sound.
Later +3B boards came pre-fixed.