I'm not going to say that Bobcat never made a "5th Wheel" attachment for a 643 but I am unaware of one. The only 'factory' made ones that I'm aware of were designed for the older 500/600 series machines. Without a pic I can't tell you what you have. There were a number made "aftermarket" that fit the 643/743 style backend and they bolted on as I recall. Dave Mendes for instance has made (or had made for him) a very nice attachment that fits later models. If by chance you have the factory made built for the 610 style back end….I doubt it will mount to the 643 but again….without a photo, I can't say for sure.