https://en.wikipedia.org/wiki/Cable_grommet
With this type of grommet, it’s primarily purpose is to act as a cable restraint, to try to stop the cable from being pulled out of the case. It’s secondary purpose is to help protect the cable where it exits the case, to add extra mechanical strength, to reduce the likelihood of the cable breaking where it exits the case due to being bent at a sharp angle. And lastly, the PVC insulation in the cable may react (a chemical reaction) with the ABS plastic of the case, the grommet acts as a barrier.
Back to the “thing”.
Looking at that better photo, it looks like there is thin wire wrapped around the outside. It may be some type of fuse and it’s been blown at some time. Then someone has added the wire. If it has a resistance of 1.5Ω, it may be fuse wire.
Low current fuses and low current fuse wire can have a measurable resistance.
Anyway, this is most definitely not a standard or original part. Someone has added it. This may explain why the circuit board is so badly discoloured where the diodes are. I suspect at some point in the past, the output has been shorted out, overloading the PSU. This would cause the transformer and the diodes to get hot (a lot hotter than normal).
Alternatively, this is some kind of homemade resistor, with a resistance of 1.5Ω, it will drop about 0.9V when a current of 0.6A is passing through it. Hence slightly reducing the amount of heat that the 7805 voltage regulator in the ZX Spectrum has to dissipate through the heatsink.
Although there are better ways of doing this.
In the past, some have added two diodes (wired in series) between the PSU and the the ZX Spectrum to reduce the voltage. Each diode drops between 0.6V and 1V (depending on how much current is flowing).
Now that modern DC/DC converters, also called switching regulators are available, one of these can replace a 7805. Because modern DC/DC converters have an efficiency of better than 85%, they don’t produce any significant amount of heat, so don’t need a heatsink. Examples are the pin compatible devices from Recom or Traco Power.
However, because they are efficient, they use significantly less power. Less current is drawn from the PSU. With less current being drawn, the output voltage of the unregulated PSU will be higher. If it goes above about 11.8V, there is a risk that the original Sinclair DC/DC converter circuitry (TR4, TR5, the coil and the other associated components) may not work correctly. The result is that the -5V supply to the 4116 (or equivalent) DRAM goes out of specification, or is lost completely.
Later versions of the Sinclair DC/DC converter circuitry were subject to changes in design to overcome this problem. Hence the different circuit design in later issue boards.
Mark
Standby alert 