I always thought of the numbers as paired with the Firebirds and just never made a bass VI or Bass VIII? It's a shame because I would love a deluxe bass if they ever put one in to production.
I & II - Single pickup, Dot inlays Guitar & Bass
III & IV - Dual Pickup, Dot inlays Guitar & Bass
V & (VI) Dual Pickup, Trap Inlays, Neck binding Guitar (no bass ever produced)
VII & (VIII) 3 pickup, trap inlays, neck binding Guitar (no bass ever produced)