An issue ...
By numbering early exclusive models 9511, 12, 13, 31 and 33 within what was to become the core range numbering system, Hotpoint effectively wrote off any full ranges ending with the numbers 1, 2 and 3, hence the jump to models ending in 4.
The numbering of late new generation machines in the next generation numbering range also messed things up a little - see the 9550, 9570, 9571 and 9900 in the other thread.
Further exclusive models would in future be numbered in their own 957X number range, started off with the 9570 and 9571 and later, when this filled up, the 977X range.