I posted a similar question 6 Feb 2014 under "General Philatelic/Identify This?: Help ID and Postmark.
I am not sure how to direct you straight to that posting, but if you search under the above listing posted by mbo1142, you may find part of the answer you are looking for. Do not know what the letters stand for, but the triangle postmark is explained fairly well.
mbo1142, thanks for the reference. That posting dealt with several issues but the crux of the triangle comments was that they were machine generated cancels for mass mailings from as early as 1902. They would seem to be collectible, perhaps I will offer to a precancel specialist.
In the post there are some who refer to these as bulk mail cancels. This is a mistake. These are Printed Matter cancels. You will usually find them on low denomination stamps and covers because of the cheaper rate for mailing printed matter.
Silence in the face of adversity is the father of complicity and collusion, the first cousins of conspiracy.. 29 May 2018 12:07:16am
re: Identify UK Cancels
There were also Inspector's Marks"
a triangle with either a number or initials
of either the inspector or the place he
operated from.
While a bit more involved, both Land's End
and John O'Groats used a double lined Triangle
with the name inscribed which could be
considered a cachet or a cancellation,
depending how it fell on the stamp or cover.
There were wavy lines included as a killer.
So if a stamp were soaked from the cover
and only showed the decorative triangle,
it might be a puzzler.
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