I guess this is the Spectrum Kempston Centronics cable?
All the ones I have seen are just a straight lead and it's the same used on most other interfaces - such as those for the QL - I think the LPRINT III cable will be the same.
Finally I got the cable and connectors from China ($ 8.31 total). After I inspected the GND pins on interfaces for orientation, there was no problem to properly connect the flat cable and connectors.
I tested it on the old Samsung SP-0912 dot matrix printer- works as a charm