Male thread portion (47) extends from a frame support section (12) so as to be capable of fittingly passing through a vertically elongated hole portion (53) of a protective cover (41) and through a through-hole (62) of a plate member (42) with fastener members (43 - 46) inserted through adjusting holes (56 - 59) of the protective cover into a plurality of holes (33 - 36) formed in the support section. The protective cover (41) and the plate member (42) are fixedly mountable to the frame support section by a nut member (47) threadedly engaging with the male thread portion (47) projecting out of the elongated hole portion and the through-hole of the plate member.
