Here is just one of many examples not specifically addressed in the Price Guide. John McD once told me EIN  which says, "[Backwards number] / [Number]" is meant to cover this situation. But this was years ago before some nut started collecting all of the data and finding all of the variations that are out there.
The slash in the PG is normally indicating one thing is positioned vertically higher on the dome such as in "45 CLIFF ST / NY." And it just so happens there are CREBs with a number positioned higher on the crown than a number positioned below them. These include pieces like a 16/15 and 16/16 which fit exactly with what we are used to the slash meaning.
As my research continued, I found MANY pieces of similar type which have no defined method of indicating their situation..
To differentiate these from something like a 16/15, I came up with using the symbolism 7(7) in my own notes. Since the SN looks as if the large backwards 7 was made to hide the smaller 7. I read this as "7 hide 7."
And when specifics are important, I write this as BK7(7) where the BK means "BacKwards."