A spectacular very fine mint NH lower right blank corner block of the 15c blackish purple Bylot Island stamp from the 1967-1973 Centennial Issue, with Winnipeg tagging and a plastic flow doubling of the "15" on all four stamps. The doubling appears as a vertical line just to the left of the "1" and the vertical stroke of the "5" and on the inside. The two right hand stamps are superb examples that show stronger than usual doubling, while the lower left stamp is about normal and the upper left is a little weaker than usual, but still quite clear. Horizontal wove paper with no visible mesh or ribbing and smooth creamy dextrose gum with a satin sheen. Under long-wave ultra violet light the paper gives a dull fluorescent greyish reaction, with no fluorescent fibres visible in the paper. Winnipeg tagged with three sets of 8 mm bars, of moderate strength, being clearly visible to the naked eye and spaced 29 mm & 27.5 mm apart. The ink colour under UV is also blackish purple. There is a very slight gum disturbance on the upper left stamp, which is also the weakest example of the variety. However the other three stamps are pristine NH. Unitrade values a VF-75 NH block with two variety stamps at $300, However, given that blocks with all four stamps having the variety are scarce, a more appropriate value would be the sum of the values of the four stamps plus a 50% premium, i.e ($100 x 3 + $50) x 150% = $525. The block offered here grades VF-75 as follows: Margins/centering: 50/70 Paper freshness: 5/5 Colour: 5/5 Impression: 5/5 Absence of visible paper flaws: 5/5 Perforations: 5/10 Note any vertical or horizontal line shown in the scan is the edge of the acetate strip that the stamp was resting inside on the stockcard.