A mounting plate fixed to a longitudinally extending rail is secured by a plurality of bolts in a substrate. The mounting plate is formed at each bolt with an array of ridges. An adjuster plate formed with a vertically throughgoing transversely elongated slot through which the respective bolt extends has a lower face formed with an array of downwardly projecting generally complementary to the ridges of the mounting plate and an upper face formed with a central recessed seat defined between two longitudinally extending and parallel edges at a predetermined transverse spacing. A gauge plate formed with two parallel outer edges spaced apart by a distance substantially equal to the transverse spacing of the seat edges and with an eccentric circular hole of a diameter substantially equal to an outer diameter of the shank of the respective bolt that extends vertically through the circular hole fits in the seat.