A reciprocating vacuum shuttle plate co-operates with a knife gate (112) and high-friction rollers (118,120) positioned between the knife gate (112) and an additional conveyor formed by a driven feed roller (94) and an idler feed roller (96). The high-friction rollers (118,120) form a singulator gap (188) with shoulders (184) of a vacuum-groove (186) on a sheet-engaging surface of the reciprocating shuttle plate. The knife gate (112) holds back other sheets in the stack and the singulator separates any double sheets which pass the knife gate (112). A transition tray downstream of the additional conveyor provides a transition to clamps of an endless chain. The transition tray comprises an endless conveyor belt with a floating feed roller biased thereagainst. The position of the floating feed roller can be adjusted so as to space it approximately a sheet length from a stationary clamp on the endless chain. The shuttle plate comprises upper panels one of which is a vacuum-groove panel which can be interchanged with one another on a shuttle plate frame so that the position of the vacuum-groove panel can be changed. A thumb mechanism provides resistance to falling sheets so that a forward-most sheet has reduced pressure on it.