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For rear mudguards be more specific, do you want the 3 hole type or the 4 hole type? It would depend on what rear light bracket you currently have of course.
Interesting question Alex and one I can't say for certain if it will or won't as I've never owned a 750. However the front wheels are both 19" so the blade will fit, the brace however is different as are the stays. If you still have the brace off your old guard you could in theory swap them over, just use some 5mm bolts and domed headed nuts. This would also give you the brake line clamp which is welded onto the brace on the 550. The braces should fit the forks BUT the braces aren't the same as the 550 ones, whether the 550 braces if you still have them will fit the bolt holes in the guard is a question you should ask. Depends on price, if it's a good price buy it and see if it fits, if it doesn't swap the brace over and it should then.