Identification of my hearing aid

I inherited a Phonax hearing aid from a relative who died. I have moderate to severe hearing loss according to a doctor. I had not the money to purchase a brand new expensive hearing aid. But I know this is probably a good one. I want to identify the model of my hearing aid (one for each ear) and learn about it before going to the professionals to get it fine-tuned etc.

Can you identify the exact model. It is a Phonax. The batteries are switched off by opening the battery holder. There seems to be no volume control.

I include three photographs of the hearing aid.