I imagine the closest anyone would come to an exact number would be to use the serial numbers. They are all stamped with a date as well.
I had three at one point, due to brokenness. One was gear replaced, one broken was sold, and I have another busted one.
I regret parting with the unit I did, because it had a later date stamped on it. The two I kept have the same earlier date and I would imagine this was the original production run with the majority of the systems. The later run is probably more rare and the serial numbers more helpful in determining units sold, as I can't imagine they did more than a few manufacturing runs.