If you definitely have Dexter axles and there is no reason why they would not be Dexters, and you want to double check the seal numbers provided for comfort, You search Dexter parts and find bearing kits that will give the part numbers for the seals.
Don't be concerned about the part number so much, it's the OD, ID and thickness that matter along with the number of lips. If the supplier offers one with a double lip, so much the better. For that matter you may have double lip already. In fact I would seek a double lip seal in a quality name such as Timken, SKF, and Chicago Rawhide (CR). The supplier will have a crossover chart for every seal.
BTW, there are industrial suppliers that carry Dexter axle components as well as aftermarket components that fit Dexter axles. If you have the seal in your hand an industrial supplier will probably reach in his pocket take out his caliper and take a ID-OD measurement and identify the part number without without even looking it up.
Am I right to assume that these are the same parts/numbers that would fit our Big Country 3690 (2013) which has Dexter 7k axles? The Big Country, Big Horn and Landmarks share the same frame and axles, right?
~ Alice