The 340 cipher was sent November 1969:
One year later, the Halloween Card was sent with the key and all the clues:
Front & Back
Inside
There are 3 references to the Tim Holt Comic LADY DOOM AND THE DEATH WHEEL
The skeleton is wearing a RED MASK
On the left side are the words "YOU ARE DOOMED'
On Lady Doom's Death Wheel and on the back of the Halloween Card are the words:
BY FIRE
BY ROPE
BY GUN
BY KNIFE
Researcher Jos Kirps originally suspected the Halloween Card as a clue by finding a recurrence of 'BY' in the cipher and proposed the rest would eventually be decoded.
I propose a solution to the 340 cipher by viewing the cipher in mirror and using the symbol from the return address as a key:
BY FIRE
BY ROPE
BY GUN
BY KNIFE
Wait, did you come up with this?
Steemit solved the Zodiac Killer message?
And it wasn't even that incredibly exciting?
Well, off to download some pornography. Cya later!