Basic concept as to how I believe the 340 Zodiac Cipher would have been done. It could also just be random characters. It's been solved! - https://www.youtube.com/watch?v=-1oQLPRE21o

ZodiacPOC.py

import string

def encrypt(the_input):
    start = 1
    encrypted = []

    one_e = str.maketrans(
    "ABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789?!. '",
    "74IKSQ'VR5LDBZ6J.N1GW?ETO8P UA023F9!MXHYC")

    two_e = str.maketrans(
    "ABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789?!. '",
    "W1GXAFCQ2YO7VNDZS?K5BUP9T3'I!ERML8H .460J")

    three_e = str.maketrans(
    "ABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789?!. '",
    "9TL04BN1?MW'GCY758S6 KIJOVPE2UFR!XDAH.ZQ3")

    for each in the_input:
        if start == 1:
            encrypted.append(each.translate(one_e))
            start = 2
        elif start == 2:
            encrypted.append(each.translate(two_e))
            start = 3
        elif start == 3:
            encrypted.append(each.translate(three_e))
            start = 1

    encrypted_string = "".join(encrypted)
    print(encrypted_string)


def decrypt(the_input):
    decrypted = []
    start = 1

    one_d = str.maketrans(
    "74IKSQ'VR5LDBZ6J.N1GW?ETO8P UA023F9!MXHYC",
    "ABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789?!. '")

    two_d = str.maketrans(
    "W1GXAFCQ2YO7VNDZS?K5BUP9T3'I!ERML8H .460J",
    "ABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789?!. '")

    three_d = str.maketrans(
    "9TL04BN1?MW'GCY758S6 KIJOVPE2UFR!XDAH.ZQ3",
    "ABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789?!. '")

    for each in the_input:
        if start == 1:
            decrypted.append(each.translate(one_d))
            start = 2
        elif start == 2:
            decrypted.append(each.translate(two_d))
            start = 3
        elif start == 3:
            decrypted.append(each.translate(three_d))
            start = 1

    print("".join(decrypted))


if __name__ == "__main__":
    while True:
        try:
            option = int(input("Encrypt(1) or Decrypt(2)?: "))
        except ValueError:
            print("Options are 1 or 2")
        else:
            if option in [1,2]:
                break
            else:
                print("Options are 1 or 2")

    while True:
        verify = []
        the_input = input("Only ABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789?!.' and space allowed:\n").upper()
        for each in the_input:
            if each in "ABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789?!.' ":
                verify.append(True)
            else:
                verify.append(False)
        if all(verify) is True:
            break
        else:
            print("Try again")

    if option == 1:
        encrypt(the_input)
    elif option == 2:
        decrypt(the_input)