Til A Proton Decay
The numbers at the bottom indicate the number of twists on an (appropriately coloured) strand, each representing a charge of e/3. Thus the neutron has a total zero charge. In the full triple of diagrams there are three -1 charges to make up an electron, three 0 charges to make up an antineutrino and three +1 charges to make up a proton. On the other hand, for the proton each such diagram is labelled by charges 0,1 or 2. This cannot be split up into a positron and neutrino set because that would leave three strands with a +2 charge, which certainly could not make up a neutron.