Maybe I can put some light (or maybe worse the thing) here....
I bought mine from a amazon seller, june last year, it does says professional.... all great.. 25-ft cord.... stupid power.... on 6 it was just crazy... eat pads, won't stop with almost all my 200 pounds over it... can do correction on 5 or 4.5 without a problem... but I always used it on 5 or 5.5..... great machine, I just think it vibrates a lot.... now here for the fun part.....
Here in Brazil we are apparently selling Megs and Griot's rebranded machines bought directly from China....
We have both a 500w and 900w model.... but both 220v... outside it's a Griot's, no doubt... I just don't know the insides...
DA Polisher v8
Maybe Griot's is selling there the 500w 110v model now and is still advertising it as a 840w one ?
Note that there's a 0.4kg difference in weight ? Maybe you can check if there's any weight difference between your machines Mark...
I mean... just speculation, but it could be...