Where are you located?
I see where you inquired about this on Collectors weekly and said that you think it was made for a car.
Portable desks were just that. Portable...so if you went to a country house for the weekend or overseas for a few months you were able to take your writing implement with you. During this time period, this was the means of communication. Everyone with "influence" had one. The finer made and more intricate, the more status one felt they had! Some were made for women that carried their toiletries as well as their writing items.