Help identifying a vintage Diamondback full suspension MTB
Looks like a late 90s BSO.

I also notice the boots/stanchions from the forks have rotted and dropped off.  And that you're missing the front brakes completely.

Not only that, the front rim is completely missing a rotor and doesn't even appear to have a rotor mount.   I suspect the front wheel was replaced at some point.  As such, this bike is not safe to ride.

The suspension is a spring with a damper on the inside.  The damper will be purely cosmetic and any damping capability will have long since vanished.  I found that a 60 kilo teenage rider could over-compress the rear suspension making the whole bike extremely saggy to ride.

That frame shares a lot with this late 90s Raleigh RFX 



UPSHOT  If you have a disk front wheel, rotor, and spare working front caliper/lever already, it looks like a straight forward tune up.  If you have to buy any more than cleaner and lube then think twice.  Unless its sentimental in which case its your decision.