Knock Three Times David Isaacs

Hi all, I can't find anyplace online to listen to or purchase a download of David Isaacs' vocal version of Knock Three Times.  The Upsetters' instro Version is available , but the vocal is not.  No reissues on comps, no Youtube video, no link on Upsetter Riddim Shower which seems to know about and link everything (thanks!), nothing on soulseek.  The Upsetter 7" is available for sale on a few reggae sites and eBay - and you can hear samples (Example: s.reggaefever.ch/samples/16/95/24-B.mp3)  The A or B side of the single - You'll Be Sorry  by D. Isaacs- is comped, why not the other side?  Why so elusive?  Is it labeled as something else and I'm missing it? Does it have an alternate title or is it floating around mislabeled? I'm not a collector, I'm not looking for vinyl - I'd just like to listen and enjoy.   If anyone has a lead please let me know.  Thanks.