I'm guessing it's a 651. Anybody else care to hazard a guess, or maybe can positively identify? http://www.flickr.com/photos/rocbolt/2694473124/
I think you are right, though it may be a 641. either way, it looks like it came from a stiff neck machine, not a cushion hitch machine. I say this because of the hoses and piping configuration.
Measure it and calculate the volume. That will tell if it is a 641 or 651. Then put a front dolly on it and decide if you should hook behind your D10 or D11.
651B, possibly a water buff (notice the pipe and spray "hat" on the back). No dolly the neck isn't long enough.
I would say 651B. I think it did have a cushion hitch. Something weird about this bowl though. I have seen scrapers turned into waterpulls by boxing and baffling the bowl. Look at the RR of the can above the rear wheel. It looks like a spray head. Also it looks like they may have turned the bowl into a tank. Note the large steel above the bowl. I think they may have plumbed the pump through the ejector lines.