Discussion
This issue is known imperforate on the right-hand margin with the Crown over A watermark on Cowan gummed paper which was used from November 1907 onwards. There are three types of imperforate issues known;
Not much is known about the imperforate on the right margin of the sheet issue but presumably, as was the case with most other mint imperforate Queensland 2nd and 3rd Sideface issues, a sheet or two was for some reason not perforated properly.
When perforated with the comb 12½, 13 perforating machine, the sheet was usually rotated & perforated a second time due to the insufficient number of columns of perforations on the Buncle's perforating machine. In this case it wasn't rotated which left the right margin imperforate. It appears that t two sheets were inadvertently issued with this error. Assuming this is correct, then a maximum of 24 stamps with an imperforate right hand margin can exist.
It never went on sale at a post office but instead must have come from within the Queensland Post and Telegraph Department and made available for purchase to certain unknown favoured collectors.
It is not listed by Stanley Gibbons but is listed by Brusden White in their 2023 Australian States Federal Period, 1901-1912 catalog, where it is listed as Q11ba. Imperforate at right and priced at $450 mint. It includes the following note: "Available evidence suggests two sheets were issued showing the right-hand column imperforate at right. "
Below is a census of examples seen by me. I have so far seen 17 of the 24 possible examples. I also show, at the bottom, an example that is perforated to the end of the right margin horizontally but not vertically, so it must be from a different sheet
Census
A block of 12 on gummed Cowan paper from the top right-hand corner of the sheet. Seen in the collection of John Pearson
A block of 4 on gummed Cowan paper from the bottom right-hand corner of the sheet. Seen in Spink auction no 12043 lot no 1407. Ex Alan Griffiths and now in the collection of Dr Andrew Mortlock
A block of 4 on gummed Cowan paper. Seen in Corinphila auctions in 2023. Ex Bernard Manning and E.R Slade
A block of 4 on gummed Cowan paper. Seen in Corinphila auctions in 2023. Ex James Mursell
Perforated to end of the right margin