More info from Phillip Houston on ATS Forum
Yes, a spectrum camera seems very likely. This camera has a rather small range of tilt adjustment, perhaps 40 – 50* or less, and only in one direction, unlike ordinary view cameras. It would work with an objective prism, and perhaps a long focus singlet objective. No need for achromatism in a dedicated spectrophotography system. In fact, a singlet lens and prism of the same glass would perhaps give a more linear spectrum spacing than anything achromatic. As John pointed out, focal length of a singlet varies with wavelength, requiring a range of tilt. Quarts optics would be needed to reach very far into the ultraviolet, but other materials could be used for visible and infra-red.
It seems likely that this plate (or film?) holder was part of a spectroscopy dedicated optical tube that attached to a larger telescope. The scale and long stem knob in the top photo appear designed for shifting the back to allow multiple images on one plate. A dark slide is pulled out at left. Typical practice for stellar spectrophotos of the day would have been to orient the prism and camera back so that the telescope drive could be shut off and the light would trail perpendicular to the dispersion spread, giving nice neat spectral lines and great stability.
On the bottom photo, you can see some narrow dovetails. This is consistent with fine English carpentry. Looks like mahogany and very well made.
Sam