Specifically for the discussion of the A-mount DSLR range
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There's a lot of misunderstanding about the A850 and A900. The prism cost and 98% view has nothing to do with the prism itself - the extra cost of the A900 assembly is due to an adjustable 100% mask fitted to the condensor and screen section. This is expensive. The A850 just has a fixed 98% mask with no setscrews or adjustable carrier. Apart from that, the prisms and screens etc are identical.
It's entirely possible this is an A850 flashed with A900 firmware, and there is no reason to think the shutter or mirror mechanisms will fail. Maybe they will produce more debris (sensor dust) and eventually wear out faster, but the camera should be safe to use.