I still have mine. I got it in 1981 or so. It cost about $150.00 then. It's built like a tank and took a beating, fell into the sand at Cape May but remained unharmed. Bounced off rocks at Hawk Mountain, dinged and dented but still works. Focuses by the objective lenses moving. They have protective glass plates in front of the objective lenses. That being said, they are rather dim, much like you would expect 30 year old non-phase coated roof prisms to be. And they have virtually no eye relief if you must wear eyeglasses. Edge sharpness is poor and FOV is 7 degrees. Externally, they are similar to the Leitz Trinovid, (which I also have). The Leitz, I might add, is optically, vastly superior to the Minolta. My Leitz is a 7 x 42 Trinovid. It's about a 1990 vintage.
They are a rather handsome binocular like the Leitz, but their prisms are different. The Leitz used the uncommon Uppendahl Roof Prism. I understand that the Mariner's are Schmidt-Pechan prisms. They also came in 10 x 40 format and an unusual 8x-16x x 32 Zoom, which must really have been dim!
Here's a thread that discusses them:
http://www.birdforum.net/showthread.php?t=107671&highlight=Minolta+Mariner
jjg 213, one of the posters therein, was kind enough to send me an old brochure on them.
I would guess your binocular would make a nice collectible but I wouldn't pay an awful lot for it. It's usable, but it's best use, IMO, is to use it as an example of how much Roof Prism binoculars have improved over the last 30 years!
Cordially,
Bob