A great resource for the Spectrum versions and images is https://spectrumforeveryone.com/technical/zx-spectrum-models/

Below is a quick access visual of model versions to images. All credits from the link above.

There were many internal versions of the Spectrum 128. They are all inside the same style casing.

First version of the Spectrum 128 ‘Toastrack’, found in early Investronica 128 machines. These are presumably Samsung manufactured as they have solder mask on both PCB sides. The /CLK connection on the edge connector is not actually connected to the CPU clock signal, so this must be manually added if peripherals that use this signal (e.g. DivMMC) are to be used.

Despite the specification stating otherwise, RAM banks 1,3,5 and 7 are contended, and 0,2,4 and 6 are uncontended. This caused some issues as we’ll see later.

Minor revision to correct some bugs on the Version 2 PCB, again found only in Investronica 128 machines. /CLK connection issue is still present on this revision.

The first mainstream UK-spec 128 machine, manufactured by Samsung. Its main distinguishing feature from the 6U is that it has green solder mask on both sides of the PCB, and uses similarly sourced components to that found in the 48K Issue 4S. /CLK signal on the edge connector is correctly connected on this and all subsequent issues.

However, there is an issue with the value of R137, which has the net result of causing the MC1488 keypad driver IC not to function properly due to out of spec voltage rails, and therefore a keypad will not work with this version. Since this was the first UK 128 version, this is most likely the reason that UK keypads were not sold (and today are extremely rare).

Changing R137 for a 15R resistor will allow the keypad to work on these machines. (credit to Ian Gledhill for this fix)

By far the most common variant of the Spectrum 128, this has solder mask on the PCB underside only, like earlier 48K machines. Some late machines feature soldered Amstrad 40056 ULA’s instead of the usual Ferranti 7K010E-5.

Final version of the Spectrum 128, mostly sold in European markets and can be distinguished by having a standard DB-9 RS232 connector instead of the usual BT631W socket.

The Grey +2 model was made by AMSTRAD.

Notice the tape recorder has icons printed on keys. The +2A (black) has them printed above.

The first Amstrad produced Spectrum, this was reverse engineered from a Sinclair 128 machine by Amstrad engineers in quite a hurry in order to make the deadline for availability during Christmas 1986 (this effort actually commenced before the sale of Sinclair to Amstrad had been formally concluded). It was manufactured by Amstrad’s contract manufacturer (Orion) in Taiwan.

As such it is functionally identical to the Spectrum 128 machines, with the addition of onboard Sinclair-compatible joystick ports (controlled by and Amstrad 40057 IC, which is essentially a MT62001, found in the Interface 2).

The PCB quality is notably poor on the Amstrad-produced Sinclair PCB’s, requiring great care when replacing components to avoid lifting pads or tracks.

A slightly later revision of this board is marked ‘0500 Issue 3’, but apart from very minor layout changes there is no functional difference between the two.

UK-built versions of the Grey +2 carry this PCB, which is functionally identical to the Z70500 but uses mostly axial capacitors and other UK-sourced components (notable, the Z70700 uses the PCF1306P where the Z70500 has the Amstrad 40058). It is speculated that this was produced to use up components that Amstrad inherited from ex-Sinclair suppliers as part of the takeover.

The +2A / +2B variant come in a black case as oppose to the Grey +2 case.

There were various internal PCB versions including Z70830, Z70833 and Z70835.

Released in 1987, the ZX Spectrum +3 is similar in appearance to the +2A, but instead of the tape drive, came complete with a 3-inch built in floppy disk drive, as was fitted to the Amstrad CPC 6128.