For what you paid you got a goodt deal on the 3 boxes of shells ALONE. It would definitely be in your best interest to have a competent smith with experience in Japanese military weapons give it a once over. The metals used in many "last ditch" weapons were anything but quality steel. In the final days of the war they were melting down anything they could to form the "cast" receivers.
Even if all you ever do is hang it on the wall you got a deal on the shells. Brand new Norma rounds for the Japanese 7.7 go for $30.95 WHOLESALE. You can occasionally find them for less from somebody just wanting to get rid of them. At the price you paid I'd almost wonder about the ammo being reloaded.
But if they are the real deal, heck, a box of 20 Norma EMPTY CASES from Midway costs $15.74. If you look at it that way, you grabbed a piece of history for $7.00 and got the loaded ammo at less than 1/2 the normal cost.
I would have paid what you did in a heartbeat, even if I thought the gun was a poorly constructed "last-ditcher".