The 550 board is, if I recall, identical to the Color Classic II and is easily swappable. Keep in mind the 520 and 550 use the 68030; the machines mentioned in the original post all use the 680LC40; if you need that newer processor for some reason, steer clear of the 520/550. Also worth noting is that the LCIII and LCIII+ are to the 520 and 550, respectively, what the 475 is to the 575 in terms of specifications. (The pizza box LCs are the "400 series", as evidenced by the related Performas; 500 series are the same thing but with integrated monitors).