Some interesting additional information about this stamp from http://www.tuvaluislands.com/stamps/st-d1979c.htm
This value was produced in order to have a stamp available for the new first class airmail rate to North America and Europe, which came in effect on January 1, 1981. Initially, 17,000 50c stamps were sent to the printers in order to apply the overprint. That is all that was available, as sufficient quantities of the 50c value had to be kept for other current rates. At the same time, the 50c stamp (along with several other values) was ordered reprinted, and 60,000 of them were requested with the additional surcharge. These took longer to produce and had to be shipped later.
The initial overprinted stamps were done by typography, and were placed on sale only at the General Post Office in Funafuti, for postal use. The Postmaster did not consider supplies large enough for philatelic sales. The first day of sale was January 19. The second printing, which had the overprint applied by lithography during a complete new printing of the stamp, was not available in Tuvalu until February 26, 1981. These were sold only through the Philatelic Bureau.
By April, the Philatelic Bureau ran low on gutter pairs, cylinder blocks and other configurations from the complete sheets. They were able to acquire about 260 of the typographed varieties from the Post office, in order to meet customer demands. It must be pointed out that at this time that the Philatelic Bureau (and stamp collectors) were unaware that there were two different varieties. They were not identified until well after the stamps were replace by a new 45c design in June.
The identification of the 2 types is also from the above site, I have used my own images to assist as the images on the site are unclear.
Non-uniform black ink, blurry around the edges. This is especially noticed in the bars and the word 'CENTS'
Slight indentation of the overprint through the back of the stamp
Uniform black ink, very sharp edges through all portions of the overprint
No indentation of the overprint through the back of the stamp
The information goes on to estimate the quantities sold.
The Philatelic Bureau announced that the total quantity of 45c overprints sold was 60,383. This of course includes both varieties. Since they received some 260 full sheets of 38 of the typographed stamp from the Post Office (10,000 stamps), that would indicate the Post Office sold about 7,000. Most of these would have been used for postage by the natives, and any remainders destroyed. How many of the Bureau's 10,000 were sold is unknown, but since they acquired the additional sheets for breaking up into gutter pairs and cylinder blocks, it must be assumed that most of these would have been torn out and sold as such. Any loose remainders could have been sold as singles or used as postage by the Bureau, but the probability is low. Most standing order accounts would have already been filled with the lithographed version.
The following collectable configurations can be obtained from a single sheet of 38:
1 sheet of 4 plus 2 labels (4 stamps)
1 sheet of 2 (2 stamps)
1 cylinder gutter block (4 stamps)
6 gutter pairs (12 stamps)
The above configurations account for 22 stamps. Assuming they all sold, multiplied by 260 sheets would total 5720. Allowing a conservative amount of 1280 singles, that would give a maximum total of possible mint typographed stamps in existence at 7,000. Many collectors may even be unaware they have this scarce stamp in their collection.
Postally used covers with the typographed overprint are very rare. At least one commercial cover mailed from the Funafuti GPO is known. It is used on an Aerogramme with an enclosure, paying the 45c rate, dated May 1, 1981.