The bottom row is always filled. The remaining 3×2 grid encodes the lowest 6 bits of the character's ASCII value in binary. You can read the columns from left to right and match them to the equivalent positions in the binary string.
For example, V = 86 = 0x56 = binary 010110 = 1st row left empty, 2nd row left filled, 1st row middle empty, 2nd row middle filled, 1st row right filled, 2nd row right empty.
The character which would be a question mark (all positions filled) is used for the start and end blocks.
Updated 09 Nov 2018. Comments/questions? Contact mjb on Discogs (login required).