Bent as a nine Bob note

I know what it means bent=crooked a nine Bob note never existed it was a ten Bob note which is really a ten shilling for some reason a shilling was know as a Bob so if u had a nine Bob note it was counterfit so it was crooked or bent this phrases was used to express a fear of something being illegal

that all I know would like more input thanks

There's an expression in the U.S. -- queer as a two-dollar bill. Meaning there is something "off" or wrong. This sounds similar to the above.