This is a nice view of the grating mechanisms. The grating itself is the block at top left that rotates through the mounting plate. It is constrained on either side by bearings, and pulled upwards at the top end by the coppery coil spring at the top. Looking now over at where the shaft comes into the dewar wall, there is a shaft copling, a wafer spring coupling (allows flexing), a pillow block holding a bearing, a clutch, a pinion (inside box now), a larger gear, another pinion (out of box) that drives a sector gear, which is attached to the grating (seen better here).
The whole point of these mechanisms is to allow precise control of the grating angle, which determines the wavelength the instrument is tuned to.
Next Picture ("The Tophat")