The GP Triumph in post number 8 is an interesting beast. Normal triumphs have splayed out exhaust pipes, these ones go out the front parrallel. Up until then they had iron head and barrells but these are alloy. During the war triumph made an engine for the lancaster bombers to run the generator for the electrics, the put alloy head and barrells on it, straightened the exhausts to narrow it up and all. On the side of these barrells there's a cast in alloy block half way up, some are drilled some not on bikes. The genuine GP triumphs are not drilled but if they were generators on the planes they were drilled and an air directing plate attached for cooling. There are plenty of replicas around as the generator sets were not uncommon and ripping bits of them was an upgrade for your bike. NZ's Dene Hollier, of car racing fame, got fifth in the 1949 IOMTT on a GP, the factory gave him the bike.
He then put another engine, a 350 AJS from memory, into it as he wanted the same chassis for the 500 and 350cc class. This pissed off both AJS and Triumph at the time although to be fair triumph didn't have a didn't 350 race bike then