Here's how they worked it out.
Dating your bike:
Eg, NH44629
First letter...
N > Nottingham
Second letter... month
A - JAN
B - FEB
D - MAR
E - APR
G - MAY
H - JUN
K - JUL
L - AUG
M - SEP
N - OCT
P - NOV
S - DEC
2,3,4,5,6 etc = the year 82, 83 and so on