Well your Shotgun is certainly in excellent to like new condition! The Magnum receiver is a bit heavier and larger than a regular A5 is and along with the "Deer-Slayer" type barrel it would really appeal basically to Deer Hunters, not bird hunters. Judging from the color of the furniture I'd say it was part of one of their last Belgium runs as the earlier ones were usually French Walnut and of a different color and grain. It also has the square knob which is absolutely fine, but many Browning purists prefer and will pay a premium for the more sought after round knob. Browning made both versions at the same time as everyone prefers different configurations.
I do question if the case it is sitting is original. Normally those cases were made for Browning Superposed Shotguns with multiple barrels, not A5's. I know some owners sought out Browning "luggage cases" and use them for other Browning shotguns - not uncommon. I suppose it could have been purchased separately while your Dad was buying the gun. I could also be incorrect and maybe it is the original case - stranger things can happen and while I know quite a bit about Browning Shotguns I am not a Maven on them. It could have been an option at the time.
Either way, the real value of this gun is its pristine condition! I know some gun owners & collectors will pay a premium for guns that are like new and prefer to put their own beauty marks on them - kind of like me, lol. Off the top of my head I'd say you could probably get between $1,500 and $2,000 for it - because of its condition. I have seen them listed for as much as $5,000 but IMHO those are just listings waiting for someone who is either a Sucker or unfamiliar with these SG's. Good luck selling it - I believe your market is with Deer Hunters.
BTW, my two favorite 12 gauge shotguns are the Belgium made Browning Superposed and the A5 - I own both. I think the word of them!!!