I think you are totally correct in your appraisal. It has all the bit and bobs needed to be a slotting head I have a feeling it was possible for a horizontal with vertical head facilities like the old Adcock and shipley jobbies.
It well may be worth an ask to the original machine tool manufacturer as they do both still exist. They may have an old boy there that may be able to Id what model its off.
Just Google xxxx machine tools and you will find them with ease.