I was looking for information on this St. Helena cancel, and found some slightly conflicting information, so I thought I'd post what I found.

Both Scott
Classic and SG
British Empire have notes, but no pictures, regarding the remainder cancels applied in or around 1904, when stocks of older stamps were sold off to the philatelic community. The cancels spanned two stamps, and were made up of purple/violet diagonal lines forming a grid (or diamonds, if you prefer). Both catalogues say that these remainder stamps have little value, and they don't count as used examples.
Where the catalogues differ is that this stamp appears to be Scott 7/SG 44, and Scott doesn't include this stamp in the list of stamps with the remainder cancels. So, if you've never seen the cancel, you might think Scott's note doesn't apply, and that this is a postal usage.
I think I'm reading the catalogues correctly. For what it is worth, both catalogues also note that the cancel was easy to remove, and that this was often done.
Now, every cloud has a silver lining...these early St. Helena issues have been extensively forged, so you'd think that if you have a remainder cancel, you have a pretty good chance of having a genuine example of the stamp printer's craft. I trust no forgers were forging the remainder cancel, too, but I suppose you never know...