Oh, I had no idea, the 16500A looks much more modern, has a color screen, touchscreen, the 1660C in comparison has a monochrome CRT without touch.
The monitor in the 16500A looks quite nice really, Trinitron? One of those could be handy for retro computer and game projects.
Think of a 1660A as a portable version of a 16500A mainframe with a 16550A analyzer module (or with and extra POD pair on the 16550A module). The specs are pretty much the same. The 1660A trades the color CRT of the 16500A for a monochrome CRT in a smaller and lighter package. The 16500A has two floppy drives as it needs to boot from one (typically the one in the back) leaving the other floppy drive free for user configuration and data files, while the 1660A boots from flash ROM and only has one floppy drive.
In a similar way the newer 1660C is a portable version of the newer 16500C mainframe with a 16550A analyzer module. Both are upgraded over the A models with hard drives, network interfaces, and PS/2 keyboard and mouse instead of HP-HIL.
The 1670-series are somewhat portable versions of a 16500C mainframe with two 16555A modules installed, or lower capability 16555A modules installed (less memory except the 030 option 1670D, or the 1670E, slower state speed on the 1670A). There were also 1670G options that added the equivalent of either a 16532A scope module or a 16522A pattern generator module.
The final 1660E and 1670E and 1670G models upgraded the monochrome CRT to a color LCD when they got to the point of being able to fit those in and maintain the same overall size and weight (or lighter). Still no touch screen.
The 16500A CPU boards used a 68000 CPU with either 1MB, 2.5MB, or 4MB of system memory. Not all modules are supported with the earlier lower memory CPU boards. The 16500B and 16500C CPU boards updated to a 68030 CPU and SIMM memory slots. Except for the 1660A series, the 1660 and 1670 series CPU boards must be similar in architecture to the 16500C CPU boards with different layouts, and without the SCSI interface for the 16505A.