An excellent reference to all of this can be found [link=http://www.mabuchi-motor.co.jp/en_US/technic/t_0100.html]Here ( click me! ).[/link]

Mabuchi manufactures most of the brushed motors used in mechanical devices, and then adapted for RC flight.

Their nomenclature ended up being a norm in the RC world, until brushless took over.

The 340, 400, 450's, 550's, 600's references all came from their naming...