The Specimen £1 overprint was first included in collector’s sets about April 1944. The overprint was made on blocks of 20 stamps; four printing were made totalling 2,860 stamps. Dangerous forgeries of the £1 exist of this overprint, and the stamp should only be purchased with a certificate.
It is believed that only two multiples exist of the £1 overprint exist, a block of 4 and a pair, which presumably came from the material sold to the Miller brothers, who were Melbourne stamp dealers.
Other multiples exist which have been defaced by black ink; these are believed to be rejected stock which was among the material stamps illicitly removed from the Note Printing Branch in the 1940s.
The first stamp on the left is from the John Ash printing, the middle stamp is from the W.C.G. printing and the £1 also from the John Ash printing. During the printing of the £1 specimen, stamps from the McCracken printing were added, there is believed to be less than 200 overprinted making the specimen very rare.
Of the 2,860 £1 Specimen stamps printed, less than 900 exist.
Specimens cannot be used for mailing as the overprint renders the stamps unusable, though I have seen such stamps with cancellations.
The above £1 specimen is rare and retails for nearly $2,000.