a little help here please, I'm curious as to how this works

Rb435G ; Rb 750

now i know that the 'G' means it have gigabit ether interfaces
i know what the '35' and the '50' means (3 ether, 5 MPci);(5 Ether, 0 Mpci)
but what does the '7' and the '4' represent

now i know that the 'G' means it have gigabit ether interfaces
i know what the '35' and the '50' means (3 ether, 5 MPci);(5 Ether, 0 Mpci)
but what does the '7' and the '4' represent

all correct - "4" and "7" indicates series of boards, for example all RB4xx boards have something common

so that's it, nothing else to it except that they liked the number at the time