Ever hear that Johnny Cash song called "One Piece At A Time", about a Cadillac a guy built by slipping one piece at a time out in his lunchbox at the factory over the years? This is the Arctic Cat equivalent of that. It's a hilarious song, I highly suggest you YouTube it.
It's impossible to tell without seeing it in person but this is/was most likely a 71 Whisker or Prowler with the Sachs engine mount removed and the base plate added. The base plate looks to be the correct length ending at the foot pegs, so it's possible it was an SSSCAT II, Climber or Screamer frame, but the jack shaft base mounts are missing. The rear wheel setup is Whisker/Prowler all the way and I can see from the angle of the chain that it's got a small rear sprocket on it. No lighting either, also the sign of a bare-bones Whisker. Without more pics, it's impossible to be sure but it's definitely a frankenbike. You can't bolt a Screamer tank on it and call it a screamer. When the owner says it's 99% original, it's clear he has no clue.
You know what it DOESN'T have? A foot brake!!!