Here is the table of values for NGK spark plugs. not all of them have the - number on the end.
The breakdown is like this: [R]  [H] [S] - 
Six fields. Some, e.g. the second field, are optional.
Some fields may have multiple letters.
Field one: Thread diameter.
A = 18mm B = 14mm C = 10mm D = 12mm
Field two: Construction.
C = hex size 5/8" K = hex size 5/8 with projected tip (ISO)
M = compact type P = projected insulator type
R = resistor SD = surface discharge for rotary engines
U = semi-surface discharge Z = inductive suppressor
Field three: Heat Range.
2 = hot, up to 10 = cold. There's no 1, I guess.
Field four: Thread reach.
E = 19mm F = tapered seat
H = 12.7mm (1.5") L = 11.2mm (7/16")
If this field is blank, an 18mm diameter plug has 12mm reach,
and a 14mm plug has a 9.5mm (3/8") reach.
Field Five: Firing end construction.
A, B = special design (no details given)
C = special ground electrode
G = racing use
GV = racing use V type
H = half thread
K = 2 ground electrodes for certain Toyotas
L = half heat range
LM = compact lawn mower type
M = 2 ground electrodes for Mazda rotary engine
N = special ground electrode
P = platinum tip (premium)
Q = 4 ground electrodes
R = delta ground electrode for BMW
S = standard 2.6mm centre electrode
T = 3 ground electrodes
V = fine-wire centre electrode, gold palladium
VX = platinum tip (high performance)
W = tungsten electrode
X = booster gap
Y = v-groove centre electrode
Field Six: (after the dash) Wide gap.
8 = .032" 9 = .036" 10 = .040"
11 = .044" there is no 12 13 = .050"
14 = .055" 15 = .060" 20 = .080"