Karen,
I doubt it's a work permit.
Only a foreigner would need a work permit, and not even that would have been necessary at the end of the 19th century (from around the 1860's until WW I very, very few countries demanded things like passports and work permits). And a work permit would of course be issued by the authorities, not a private company.
It sounds more like a testimonial. It is still the practice here that a testimonial is written when you quit a job. This applies particularly to simpler jobs - you'd expect someone who's worked at a supermarket check-out to present a testimonial when applying for a similar job somewhere else, but not a surgeon, a CEO or an IT professional (they'd be headhunted or, if actually applying for a new job, they'd give references - names of people recommending them).
The only permission possibly given in such a testimonial would be that your previous employer might state that you have indeed quit your job, have no further obligations towards the company and are free to look for work somewhere else (a sort of release clause).
Ingela