what are "letter corners" for?  (those small soft metal things which
clamp round the corner of...letters or paper)

hi, jas ~ those cool little gizmos are to protect the corners from
being crimped or folded, torn or turned down; usually they are to
protect signed original documents, contracts, or photographs [as in an
album]. you can find them in most stationery or office product stores,
if you want some; they come in hard stock/paper, metal and sometimes
even plastic now.

i don't know where they originated, perhaps sometime in the early ages
of the wide availability of paper products, but before the advent of
photocopiers and computers?

no search strategy used; i work in the law and just know this from
personal experience.  you don't really see them much lately, which is
pretty sad now that i think of it - they are rather cool looking, and
add a touch of elegance to an original document, kind of like
gold-foil seals and ribbons, wax seals, stamps and blue-back covers
used to do.

Hi Jas,
These "letter corners" are often used by scrapbookers to enhance
documents or photos - I've often seen them in scrapbooking specialty
stores whilst browsing with scrapbook-obsessed friends. :)
Emjay-ga

- so, they're mostly for decoration and some protection. 
I imagine that they would also be useful when inserting a corner of a
'scrap' or picture into those diagonal slits used in photo albums
(maybe this is actually their primary purpose!)

Thanks all!