This place seems to be a decent selection for you. I can't say if they have the right person to answer your questions, but they are a better chance than simpler gun shops. You could call them first to see who to talk to but you'll still have to go in for a correct diagnosis. You could try another shop for a "2nd opinion.
I also needed information regarding an old German firearm to help me decide whether to keep for the uniqueness or sell for the money. Everything has its price, and I didn't keep the gun. Each employee has a specialty, a specific knowledge of the type of guns in the department of which they work. Walk in, be prepared to be pleasantly shocked by the inventory, and either browse the showcases or ask where the department of your gun type is. I bet you choose to browse. Oh, they also sell/buy knives.
Take pictures and email them along with any serial numbers or markings from the shotgun. With those, they could probably tell you everything that you really want to know, at no charge. If you want to sell it, or have it inspected for function or maintenance, then you would need to take it in.
If you need information about it you have to go to the store or send a bunch of detailed pictures to them.
I strongly recommend to directly call the store or go in person.
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