I am practicing receiving Morse Code with Koch's Method and Farnsworth Timing. As I increase speed, the places where I am running into trouble are distinguishing B from 6 (both are a dah followed by a few dits) and distinguishing V from 4 (both a few dits followed by a dah). I think I don't have a similar problem with 1 vs. J, for example, because the series of dahs results in more time for my brain to process the character. And since there is no dit-dit-dah-dah character (at least I don't think there is in the software I am using), it is easier for me to recognize "2" as two dits followed by lots of dahs.
I realize that 99.999% of learning the code is just practice and repetition. I just wanted to see if anyone had any tips for B/6 and V/4. Someone taught me mnemonic devices for "?" and "L" which I found very helpful. I was curious if there are any tips for the characters mentioned above.