Thanks for the link - I'm familiar with this BNAPS Guide, but had missed that one line explanation.
In 1924, panes of the 1’ yellow, 2’ green, and 3’ carmine were issued imperforate. The imperf panes were not distributed to post offices. They were available from the Philatelic Agency, and some were released by favour.
However, this sort of begs the question why these stamps warranted catalogue numbers if they were never distributed to post offices and are essentially philatelic in nature.
Interesting, thanks again.