Guess the likes of Rigby & H&H did use other makers actions & 'customise' them so to speak to the individual buyers preference & taste.

If it has an authentic Rigby's mark/address on the barrel then it could be a Rigby....albeit a more PH type (should still have a very good finish, smooth action, perfect feeding etc) rather than the fully engraved & extra fancy wood show ponies like the top end jobs?

I'm a no expert what-so-ever in any field so please correct me if I'm wrong fellas, but isn't that really the only way you can determine such a firearm (apart from records if they're still available of course)?
If they wanted to truly sell with the "Rigby" connection then the seller should supply the consignment note/Rigby journal entry that goes with the rifle to do so.
Apart from that there is nothing wrong with the old PH actions they are solid & dependable!