If you look closely at pic 8 you can see the groves in the flywheel for the v belts.
Looking at both pictures 1 and 8, all I can see is the feint mark left on the surface of the flywheel rim where the 3 Vee belts were running on the surface of the flywheel riml AKA flat belt fashion.
I would speculate that at some time in this engines history, that the original flywheel had worked loose on the crankshaft (this is quite a common occurence on single cyl Lister startomatic sets), hence on a twin cyl set its even more likely do to the greater load required by the alternator.
Further more, the seller states the set is rated at 7.5KVA. As the engine can only produce 10 HP the alternator is far too big for the engine. As a 6-1 startomatic would be rated at 3KVA and a 8-1 Startomatic rated at 4.5 KVA.
Hence this set should have a 5KVA alternator fitted to it.