Amstrad only made 4 versions of spectrum, of which two have an identical motherboard.
The Spectrum 128 (aka Spectrum 128+ or Spectrum Toast Rack)
(notice how it looks like a slightly elongated spectrum +)
This was the last "Sinclair" produced Spectrum, it featured 128KB of RAM via bank switching, it also provided an A/Y synth sound, RS232 serial or AUX, RGB, RF and audio out port. (also has ear and mix inputs).
Now the first Spectrum Amstrad produced was the Spectrum 128 +2
This classic grey model with tape deck provided all the features of the 128+ however there were known errors on the motherboard in regards to the composite signal (a transistor is installed in the reverse manner)
The 128 +2 A/B
This black sleek spectrum has more in common with the Spectrum +3 than the +2, in fact so much so it has a virtually identical motherboard to the +3, as such it is considered a backwards re-work of that model (except the floppy disk controller of course).
with a change of ROM and a couple of motherboard fixes the +2 motherboard can use CF/IDE 8/16bit hard-drives drives =)
Importantly, the A and B revisions only mark moving manufacturing plants within Asia. some people get the A/B revisions confused with the Grey and Black +2's when both revisions are black =)
Now the Spectrum +3
This is a nice bit of Spectrum awesome! like the +2A/B it has a RAM disk but sports a spiffy Floppy Drive, - although its the ghastly 3" disks you can happily add a 3.5" floppy drive and by updating the ROM to +3e roms you can enjoy full 720kb CPM action =)
The +2 and the +3 can have a ROM upgrade that allows them to use an internal Hard-disk (yes how cool is that), pending the ROM you can lash-together a very simple 8bit ATA interface or use a CF card in 8bit mode for full size support.
I have a bit of a spectrum collection -
1. my Spectrum +2A will have an internal CF Hard-disk in 8bit mode, and an external FDD
2. my Spectrum +3 will also have an internal CF hard-disk in 8bit mode, and will support 3.5" FDD and a HxC hacked into the chassis.
3. my Spectrum 128 +2 has the DivIDE 16bit ide interface =D
4. my Spectrum 128 + (toast rack) has an interface 1, microdrive and RamTurbo
5. my Spectrum + has Kempston interface and a portable CD player for Turbo Loading =D
I may yet get a spectrum 48k rubber key for completion of the collection, I am also tempted to add 8bit CF hard-disks to 4 and 5.
btw, I have no idea why I have so many Spectrum's, I hope to have them up for display 'n play at Blackpool R3Play this year =)