GB 1912/21 KGV 6 P No Watermark?

I was sorting KGV deffinitives, separating 1912 and 1924 issues and found a 6 pence with no watermark. Michel lists it (Mi #135 W), but I didn`t find any more information (in my older Zummstein and Y&T catalogues this variety is not listed).
Do you have any more information?
The first stamp has no wmk, second is from 1924.

The preface of Stanley Gibbons' British Empire catalogue states that single watermarks that are missed are not listable, and multiple watermarks may be laid out so that certain stamps within a sheet will not have a watermark; again, they say they are not listable.

Note that these comments are general in nature and not specific to your particular issues. There is no specific comment about a missing watermark for this issue in my SG.

[edit: I don't see any mention in the Five Reigns Commonwealth catalogue, either.]

One big thing I noticed and want to point out is that the perfs don't line up.