Did you receive single self adhesive stamps from the philatelic bureau in 1998?
If the answer to this question is yes and they are still in your collection take a few minutes to check the perforations.
Look out for unusual pointed like corner perforations as are classed as an error.
This pointed like perf error is now in demand and is retailing at £200 + per pair.
The error went unnoticed for four years, until they took the interest of a keen eyed collector who bought them to the attention of others.
These 2 key stamps SG2039a and SG2040a were only issued to account holders by the Philatelic Bureau in single form , they were not available from any other source. The stamps were guillotined from sheets into singles at the printers, presumingly to make it easier for Bureau staff to distribute them to account holders who had singles on order.
For this reason complete sheets, multiples and top panes do not exist.
Now known to specialist collectors as "Dagger Perfs"
they were printed in gravure by Walsall Security Printers and issued on 06/4/98. It is estimated that only about 400 pairs have survived so the catalogue value rests at this time at £300 a pair.
I had 20 pairs of these singles in my collection, but sad to say none of my copies had the error perf, just my luck ! However yours may, so take my advice and do take a look.