Inline six fuel pumps?

[balmoralbob]

Am rebuilding a '68 1250 Merc and have just checked parts inventory for fuel pump hoses for this model, it shows 2 x fuel pumps where mine has 3? This is apparently not unusual as these blocks were used with a lot of different motors. Does anyone know why this happened and the benefits of it? Seems rather odd and difficult to obtain original parts.
Any info appreciated
Steve