This appears to be a Bausch & Lomb precision grating of a size commonly found in Jerrel Ash instruments, however the label is missing and it came without any holder.

Grating size - 60 x 60 mm
Focal length - infinity
Grooves / mm - unknown
Blaze angle - unknown
Catalog number - unknown
Coating - Al (?)

This grating has numerous blemishes in the coating and a few very light scratches. Obviously only suitable for rudimentary experimental purposes.

Photo set 1656