Thanks, Gary, Peter will appreciate the plug. However, I am not the Gutter Pairs as most everybody knows. I am careful to keep myself separate from his business. However, he did want to ask the question, so I agreed to post the question for him.
There is a number of known varieties of last letter of “RUB” (broken “B”) on this stamp. Liapin lists some of them, but the list in not complete.
There are copies of this stamp that instead of “РУБ” have “РУВ” (overinked), “РУЕ” (broken bottom of letter “Б”), etc. I guess your example is one of those. I don’t think it is overprint cliche variety. Rather piece of perforation that got on ink on several sheets.