As Kurt said, AN stands for Army/Navy. The AN standards (for many things, bolts
and washers and tube fittings are of most interest to us) date from the WW II
era.
The key difference between AN fittings and "other" fittings is the flare angle
(37 degrees instead of 45) and the backing nut.
British cars usually seal hydraulic lines with a bubble flare - this is an
almost double flare - backed with a flared nut. The nut gets tightened,
pressing and turning against one side of the bubble, and sealing with the other
side of the bubble.
American/Japanese cars seal lines with a double flare - same principle but
different shape - the nut turns on a different surface than the sealing
surface.
These systems both work for their design center: putting things together on the
assembly line, once. The double flare, in particular, is not reliable across
multiple make/break cycles. Double flares are relatively hard to make
(reliably) by hand.
The AN flare is a single flare with a separate backing piece. It is dead
reliable, easy to make, and designed to survive multiple make/break cycles. It
is also more expensive.
I don't have time today to go into detail about upgrading your brake lines
(sorry to be a tease) - it's an article I've been meaning to write for a couple
of years now, and maybe this winter I'll finally do it. However, if you're
interested in learning lots more about the different flares work and what
hardware is available, I suggest you get one or more of the following books:
Dave Bean Engineering's "English Ford Racer's Catalog" - has lots of hardware
and goodies that apply to our cars, and a nice explanation of the various
plumbing systems and how they fit (or don't).
Carroll Smith's "Prepare to Win". The fundamental resource for learning to
build a car the right way, so it won't strand you.
Carroll Smith's "Nuts, Bolts, Plumbing". Updated and expanded version of the
fastening and plumbing chapters in PtW, with some metallurgy and theory thrown
in.
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