It's always been my understanding that the bump in the tire well on Manx 1 bodies was to clear the left rear brake hose bracket (where the hose & the hard line connect) that is on the swingaxle axle tube.

Have FUN FUN FUN!!!
Michael Cates - ManxManiac - the 'Maniac
Ventura, CA
Manx Club Member #958 
www.dualsportbuggies.com