After some investigation, I am pretty sure the other key is for the FFT Analyzer program. In other words:
- VP5267 key: Dual ch. Digital Filter program (Type 7667)
- VP???? key: Single ch. Digital Filter program (Type 7666)
- VP5278 key: Dual ch. FFT Analysis program (Type 7651)
- VP???? key: Single ch. FFT Analysis program (Type 7669)
- VP???? key: Zwicker Loudness program (Type 7638)
- VP???? key: Vehicle Signal Analysis program (Type 7668)
I cannot seem to find any other software that was made to run directly on the 2143/2144, but there are a few other programs that seem to be designed to run on a PC with an analyzer connected. These analyzers are also able to boot code via GPIB, so I may guess certain of those PC-programs will take use of that feature.
Besides this, from reading some of the manuals, it seems like the 2144 came default with the key for Type 7667, while the 2148 came default with the key for Type 7651. Otherwise these two models should be identical. 2145 and 2146 seems to be more specialized, being a "Vehicle Signal Analyzer" and "Sound Level Analyzer" respectively. I guess these follow the same pattern, coming bundled with Type 7668/7638 keys by default, but these may have other differences in hardware in addition to just license-keys. From a parted-out unit on Ebay, for instance, I see that the 2145 has an additional third DSP card whereas the 2144/2148 only have two.