Its not rare or unusual, in fact its the opposite, quite a common stamp, but I thought I would show it here as an example of what can be found to compliment a collection.
Prior to the beginning of 1971 certain pre-decimal 2d stamps are found with overprinted names.
Government offices and certain power suppliers used these on receipts or bills. The overprints were usually in the form of names or initials of the office concerned.
Fiscally used stamps normally have a hand written signature added or are date stamped, some stamps with just the overprint similar to the one left can also be found where they have been postaly used.
These are more sought after and are rare to an extent, the prices they command reflect this, good mint examples can raise prices into hundreds of pounds..
The £1 pre decimal stapled stamps cooks booklet is known to have contained panes overprinted "SPECIMEN".
Five values are listed these are: 1d 2 band, 5d 2 band, 4d left band and 4d right band.
The stitched version of this booklet were also issued with a philatelic bureau CTO overprint. It has been suggested that these booklets with the CTO overprint were sent out to certain firms to attract future advertising.
Also a £1 stapled decimal Wedgwood booklet was issued with panes overprinted "SPECIMEN". Stamps listed are the half pence 2 band , half pence left band, two and half pence side band ( left or right) a two and a half pence centre band and a three pence two band.
School stamps overprinted specimen were also used by the post office to train employees, these were not technically available to the general public, but they do exist in peoples collections.