Machin Booklets - Dark Flaw In Hair Variety

Hello all Machin buffs,

The scan attached shows the difference between this flaw on cylinder 17 and on cylinder 18 SG SPEC (UB25).

On cylinder 18 the flaw is much lighter and the dark spot behind the crown on cylinder 17 has almost disappeared on cyl 18. This spot is not mentioned in my (outdated) S.G. G.B. Spec cat vol 4 (sixth edition)

Can anyone explain this? Has it been retouched? I can find no mention of this in the catalogue.

The DFIH flaw is also listed under UB25b - unknown on one perforation type and unpriced on two perf types.
This (presumably) is on a non-cylinder pane. Does anyone have, or has anyone seen, an example of this?

I have a fair number of panes with this variety on the ½p, 2½p and 3½p but they are all on cylinder panes,

Any thoughts or help on this appreciated.
Regards to all
Ron