Barthel (and Fischer) used another convention with four different letters:

a   presumably the first side of the tablet

b   presumably the second side of the tablet

r   certainly the first (recto) side

v   certainly the second (verso) side

I do not like this convention, because sudden insight might lead to the conclusion that the text on a side which until now has been considered as 'a' should be 'b' instead, or worse - certainly must be 'v'. My own convention, with arbitrary 'a' and 'b', is neutral (carries no information about what is presumed or certain) and therefore is safe against such events. I have 'translated' to my own convention like this: