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Are you looking to sacrifice some versatility (lens selection, etc.) for more portability? If so then a rangefinder like the Fuji 670/680/690 maybe?
If you're looking for interchangeable lenses then I'd go Pentax 6x7, not small or light by any stretch of the imagination but they're built like a tank.
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Unfortunately there's no compatibility with the lenses, but how about the smaller brother, a Mamiya M645?
This should feel funny for you, coming from the huge RB..There are so many great medium format systems out there.. I think you have to decide what format you want first, and whether you want interchangable lenses.
So, my rb67 is knackered. A broken component means it focuses short.
I'm thinking I might sell all my lenses and all other bits to find a more convenient medium format system - i'm pretty sure the size of the thing contributed to its demise. It was simply too heavy to be able to pack safely in a rucksack, and must've been bashed in the process.
So - medium format suggestions under £500?