Fender's production methods from the early fifties had the effect the numbers may not be consecutive.
There is another reason why the G-303 is so popular: it is just a really great guitar.
History Fender, under the ownership of CBS, acquired the Squier brand name in 1965 when it bought a USA based string making firm, but it lay dormant for many years .
Before the Fender Squier series were introduced in 1982, Fender were making lower priced guitars such as the Fender Lead series at their Fullerton California plant.
In terms of sophistication of design and electronics, the weighty Ibanez IMG2010 comes out way ahead of the G-303, but like the Roland G-707, the IMG2010 is a bit of an acquired taste, and its curious body design means that the Ibanez IMG2010 is virtually impossible to play sitting down without a guitar strap!
The other vintage controllers, the G-202 and G-505, are well-built, fine guitars.