NSRT reads the handset information based on IMEI and you cannot trick it by flashing a different firmware. It will only show as available the correct firmware version and variant according to the original product code. Since you have created a mismatch between product code and firmware currently on handset, the tool does not offer a software package for download and installation.
This is not 100% true, I have seen quite a few 920s with firmware swaps, usually ATT 920s (RM-820) switched to firmware for a CV 920 (RM-821) in order ro get an early update (I think it was popular while ATT was dragging their feet on relasing Portico OTA). It's not super common, but it has been possible in the past to force a firmware switch using NCS and other tools. Phones in this condition are not able to download a software package for their phone in NSRT, maybe that's where the IMEI/Product Code mismatch come in to play. Too bad, how does one recovery a phone in this case?