Yes, harps had levers back then. I'm not sure quite how the mechanism worked: it wasn't the modern clarsach with hand levers (that only dates from around 1890) and the modern pedal harp was invented by Erard around 1830. But the two-stave scores of Scottish tunes published after 1780 all suggest the harp as an instrument to play them on, and they're limited in range rather than in key.