Ian....you've found one of those quirky anomolies in the rulebook. Rule 18.7.4.1 states "Yamaha: AT1*,DT1,CT1* rectangular swingarm, DT1, RT1 pre reed block"The rectangular section swingarm requirement was in the rulebook long before the RT1 was declared eligible for the class as a 'flow on' and it hasn't been removed. The round section swingarm is legal. The eligibility criteria could do with a hefty tuneup but when you condider that it was developed by Peter Drakeford and his team nearly 20 years ago, it's served us well. Common sense usually prevails where anomolies occur
* A good example of the 'mistakes' in the eligibility criteria is that although the book calls for rectangular swingarms for Yamaha AT1 and CT1 models, they were never equipped with them and the DT1 arm doesn't fit.