Abstract: A substrate support module (10) includes a series of vertically arranged guide shelves (12, 14 and 16) having front, intermediate and rear circular bosses (34, 36 and 38) projecting from opposite ends thereof. The first and third bosses (34 and 38) are tangentially received in elongated horizontal slots (40 and 44) in side support plates (18) to position and lock the shelves (12, 14 and 16) against vertical movement. The intermediate bosses (36) are tangentially received in vertical elongated slots (42)in the side support plates (18) to position and lock the shelves (12, 14 and 16) against horizontal movement. During assembly, edges of the slots (40, 42 and 44) tend to flatten or shave off small peripheral portions (50) of the bosses when the bosses are slightly oversize, whereby the oversize bosses can readily be assembled into the slots.