The adhesive on the left has a brown or brownish overprint. I had it for many years but not come across another example. The other stamp has the normal red overprint and is also a perf 15.
I have seen a couple of the 16c Hornbill stamp with a black overprint instead of blue. There is also a very rare red overprint variety that I would love to possess.
Just an observation, have you noticed that the "button" to the left of "S" at the top is placed a bit off center?
Apart from the presence of the narrow or "Numeral 0" variety, these two adhesives also has the unrecorded short or "Broken N" flaw. This is more pronounced on the right hand stamp. The "Broken N" variety is well recognised in the Brunei MBE series.
It is rather surprising that the left sided stamp is noticeably taller. One would have thought that the overprints were carried out on stamps from the same batch. Again on the left stamp, there are obvious flaws due to using a worn plate which would imply a later printing.
Update 04 May 2014
I have borrowed this image from a collection on auction.That is why it is a bit blurred but nonetheless, one could see the "Broken N" and this infers that this error should be present in the other values. It would be interesting to know whether it has a consistent position on a pane of these adhesives.
I had this all along and had wondered about it before. The "N" does look slightly shorter.