Boonton 4200 RF Power Meter setup problem

This digital RF power meter is some three generations old, but still useful and quite stable. In my application, the Boonton 4200 is paired with a twin-diode sensor rated to 18 GHz. The sensors have model numbers such as 51017 (8E). I believe the 8E designation was used in Boonton's older analog power meters and 51017 is the newer part number. Both the Hewlett Packard (HP)/Agilent 43x-series RF power meters meters and the Boonton power meters, the sensor includes a correction factor for frequency on the label. In the Boonton 4200, this correction factor can be programmed into the power meter. The Boonton 4210 uses an analog correction factor control knob similar to HP/Agilent 43x-series power meters. In order to input a sensor's characteristics, the 4200 manual states "Note the sensor number indicated on the barrel of the sensor, and enter this number through the numerical keypad..." The manual does not specify whether to enter the sensor's model number or its serial number, but this is solved simply by looking at the front panel where S/N (serial number) is shown. That's fine except for one major problem: both the model number and serial number of the sensor are comprised of 5 digits. The 4200 will accept an input of only 4 digits! The only clue I've found is in the Boonton 4220 RF power meter manual where the procedure specifies "the last four digits of the sensor serial number". That would seem to be a clarification of the procedure in the 4200 manual - but it doesn't seem to work. Has anybody else managed to solve this?