not really. x is not used with run numbers internally (at least on paddles)
x is usually used as a placeholder. for example, if a b/o has run number 021, some operators will display it as x21, 2x1 or 21x. in this case, it's used as a placeholder for zero, for any assortment of reasons, especially when it pertains to the manual run boxes. sometimes, when hitting potholes, the run box's numbers will slip, from zero to x or from 5 down to 4, etc.