white letters on ign. coils

[volgapower]

hi all,
i got a question, what do the white letters on the top of ignition coils mean?
i've seen many of them: 424, 391, 674 etc...
see the pics

http://s29.postimg.org/5z5vy0mtf/Bcty6_HQBmk_KGr_Hq_IH_Cg_Eqt_R5_PRUw_BK1t_Jvn4_w_1 2.jpg (http://postimg.org/image/5z5vy0mtf/)

http://s27.postimg.org/sgz0bjgy7/image.jpg (http://postimg.org/image/sgz0bjgy7/)

[volgapower]

bump
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:down:

[Def]

It's like a Davinci code mystery for 20+ year old Japanese car parts.

[volgapower]

is there any differecne in impedance or are they all the same?