I suspect that you now have a some electronic junk, If the battery goes flat or it is changed without the unit on power all the calibration information is lost.
If I remember correctly, and if the URV55 is like the NRVD, the calibration procedure and the requisite GPIB commands are documented in the service manual. Without the R&S service software, you could do the procedure by hand for each range and write the cal constants via GPIB manually (or create your own script).
The procedure requires a DC calibrator (also under GPIB control when using the R&S software) as well as a special pod that feeds the DC voltage from the calibrator into the instrument. You would have to reproduce that as well somehow.
I think there also is a command that dumps all calibration values to GPIB, in case you want to save them.