I suspect its the following. In the Good Old Days (tm) only MCBs were intended to go in that DB. Assuming that the MCBs can mechanically/electrically be connected either way round, then which way round to put them is completely arbitrary, and so it comes down to an aesthetic decision the DB manufacturer made when they stuck 'on/off' labels on the front of the DB. That manufacturer happened to choose 'on' being towards the central spine.
However, RCBOs aren't bidirectional; they typically have a bus bar termninal at one end, and 3+ connections at the other end (2 terminals, a N flying lead, maybe a FE) so the base has to go towards the bus bar. So in newer boards the manufacturer sticks the 'on/off' label with off outwards for consistency.
I suspect its the following. In the Good Old Days (tm) only MCBs were intended to go in that DB. Assuming that the MCBs can mechanically/electrically be connected either way round, then which way round to put them is completely arbitrary, and so it comes down to an aesthetic decision the DB manufacturer made when they stuck 'on/off' labels on the front of the DB. That manufacturer happened to choose 'on' being towards the central spine.
However, RCBOs aren't bidirectional; they typically have a bus bar termninal at one end, and 3+ connections at the other end (2 terminals, a N flying lead, maybe a FE) so the base has to go towards the bus bar. So in newer boards the manufacturer sticks the 'on/off' label with off outwards for consistency.
