Thanks x-pert and waste - think I have it straight now. 'A' is skimmer and 'B' is split. I don't think there's a 'main only', as the 12 o'clock position seems to be 'off' and the 6 o'clock position seems to be 'off' as well. Don't know if that's strange, or normal for super-old valve, or if I'm...