Despite the description stating they fit up to 50, they don't! Not only that, none of the Petzl extension bars extend to 50 as stated either! Check the Petzl website for size guide. However even they say the Flex bars are compatible. They are but are no longer than the bars that come with the crampons!
This is what other customers say:
DO NOT FIT BOOTS OVER EU45!
No, I would not recommend this product to a friend