No I hadn't seen it before. Indeed I was more interested in whether the position was legal or not, but once I worked out how the White bishop made it to b1, the penny dropped, and I realised what the 'trick' was. btw Spending the last 2 weeks as a tournament arbiter certainly helped.

How did the bishop on b1 lead you to believe that the position had repeated itself?

The bishop on b1 is the most peculiar aspect to this puzzle. The question we need to ask is how did it arrive on b1?

Its a real mystery!