Zodiac killer's 340 cipher solution

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

1. The skeleton is wearing a RED MASK
   

2. On the left side are the words "YOU ARE DOOMED'
   

3. 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