First of all forget the md418 as this is a totally different mic.
Here couple of info pages from old original sennheiser booklets regarding the md408, mds1 and the later MD409-U3, unfortunately could not find anything of the original MD409-N .
I am affraid only that sennheiser was not quite accurate at the time with this kind of info sheets, a tiny example is the info sheet about the 408 and 405 shown in the same page, while the 405 has a totally different capsule with an extra humbacking coil on top.
So looking at old schematics or data sheets will just make things more complicated.
Back on topic now..
The MD409 has the very same capsule as the MDS1,Grundig GDSM200, MD407 and Echolette ES14 ,All these microphones sound and respond identical to the MD409.
if your mic bares the capsule that is shown below then it is a 409 one , note that these MD409 capsules are markt with a 402 number.
Several ebay sellers claim that their microphones (i.e md408 , md403 and sometimes some other ones) have the same capsule as the 409 but this can not be true unless their capsules are replaced of course.
There were no "B`s" or "seconds" going to Grundig or Echolette as some people think, cause one thing is certain and thats that for sennheiser it had to be O.K no matter for whom they made it for.
Hope this helps.