What am I missing here?

What on Earth is an RW57b? Or if a discontinued Scott number, what was it?
No such catalog listing in Scott for years. Now an RW57a is an error where this back black inscription is missing:

HOWEVER that
inscription is printed on top of the gum. Scott notes in its footnote that genuine RW57a stamps must be with full gum, no gum examples don't count.
COMMENTS, OBSERVATIONS, EXPLANATIONS????