UPDATE
9 January: we have seen the stamps and can confirm that the Machin
definitives are coded M17L MPIL. The iridescent printing is difficult to
see especially on the £1.17 but this will be the only instance of this
value on ordinary gum.
The
Northern Ireland 2nd class is a very good match to the last sheet
printing we have although the phosphor is less evident to the naked
eye.
UPDATE 9 January: this sheet is self-adhesive, the only way the stamps appear on this paper.