The P80318 is the Differential TDR probe. The P80318X is the P80318 Differential TDRprobe plus an additional probe body. The advantage of ordering the P80318X is that you would have a backup probe body for when the first one needs to be sent in for service. Both the P80318 and P80318X,  are shipped with the same accessories; one set of two precision SMA cables, soft storage case, user manual, and three replaceable ground shorting straps.