NA = Natural Amber, the glass is actualy amber. Used to be made with
Cadmium, upon which there are now restrictions for environmental reasons
(it's a toxic heavy metal that's difficult to dispose of safely). Non-Cad
amber glass is available but very expensive, so most amber bulbs are now
"painted" (dip coated) in a transparent amber gookum. Some gookums are
much better than others -- off colors, burn/fade, etc. Some marketers call
the coated bulbs "A" (as in 3157A) to differentiate them from the "NA"
bulbs; other makers are retaining the "NA" numbers for the dipped bulbs.
(20 years ago, "A" indicated a bulb that was literally painted school bus
yellow. You could not see the filament through the yellow paint, which
scratched off easily. Nobody sells these any more.)

"K" = Krypton, a gas that can be added to the fill mix to improve bulb
life without reducing bulb output. Some makers use "LL" instead of "K"
(So, for instance, 3157K and 3157LL can be considered equivalent to one
another).

Now, output listed bright/dim:

3057: 32/2 candlepower (also 3057K, 3057LL)
3057A: Amber version (also 3057NA, 3057NA/K, 3057NA/LL...)

3157: 32/3 candlepower (3157K, 3157LL)
3157A: Amber version (3157NA, NA/K, NA/LL...)

3357: 40/3 candlepower (3357K, LL...)
3357A: Amber version (NA, NA/K, NA/LL...)

3457: 40/3 candlepower, high-heat base (K, LL...)
3457A: Amber version (NA, NA/K, NA/LL...)

4157: 32/3 candlepower extra long life (K, LL)
4157A: Amber version (NA, NA/K, NA/LL...)

4114: 32/3 candlepower ultra long life (K, LL)
5702KA: Amber version

The last two bulbs on the list exist because of GM's stupidity. GM voltage
regulator set points have been creeping steadily upward for decades and
are now at around 15.4v, which is causing GM bulbs to fail early and
often, especially those bulbs that are always on. The 4114 was devised as
an ultra long life replacement for 3157/4157 in GM's trucks and SUVs with
white DRLs, while 5702KA is the amber version for GM's vans and cars with
turn signal DRLs. At the moment these bulbs are available only from GM
dealers (to get 4114, ask for a daytime running light bulb for a 2003
Chevrolet 1500 pickup or a 2004 Chevrolet Express van; to get 5702KA ask
for a park/turn/daytime running bulb for a 2004 Saturn Ion).

The 5702KA is a very good long-term fix for any vehicle with turn signal
DRLs ('96-'00 Chrysler minivans in Canada, '01-up PT Cruiser in Canada,
early Mazda Miata, etc.)

As a final note, do not be tempted to replace a 3057 (32/2cp) with a 3157
(32/3cp). Doing so will increase the taillamp intensity by 50%, which by
itself might not be a bad thing, but which will interfere with the
bright/dim intensity ratio, making it harder to discern your brake lights
from your taillights, or your turn signals from your parkers if used up
front. If for some reason you need to increase the intensity of the dim
mode (park or tail) originally equipped with a 3057, then use one of the
40/3cp bulbs, but be careful to make sure the bright mode (brake or turn)
isn't glaringly bright.

DS