The 305cc YM1 bike that Yamaha produced between 1964 & 1966, (using lots of parts that were used in the YDS3), was a competitive bike against the Honda CB77 that also had a 305cc engine, (based on the 250cc CB72 Honda), that was produced from the early 60's until 1967.

Most likely simply Japanese Manufacturer competition for a certain section of the market.

Both of them good for 100+ mph (Marginally).