Re: !935-1936 issues
Not an easy question to answer,however, Firstly you need to understand the papers the Single Watermark stamps all have Vertical Mesh which means that the mesh runs verticalll through the the watermark and the Horizontal Mesh runs horizontally through the mesh.
Another although not perfect test is the 'curl' test to test the stamps are warmed in the hand until they curl a little breath helps. Stamps with vertical mesh should curl vertically [the sides curl towards one another] Stamps with a horizontal mesh should curl horizontally [top and bottom curling towards each other].A sideways horizontal mesh curls vertyically ans a sideways vertcal mesh curls horizontally.
The answer to your other question on the 1 1/2d Offical I am not sure if the whereabouts of all the 4 are known but the block of 4 was sold by Harmer in July 1962.It was then broken up a pair are certainly in the USA but I do not the whereabouts of the others. They came about because the block was patched into a sheet of 13 1/2 x 14 from the wet printing with brownish gum and the patched in block was from original dry printing with white gum perf 14 x 13 1/2
Hope this helps