Not sure how they account for sales, although they probably aren't even actually paid until the systems are installed so your guess would likely be correct. However, in the case that they are paid in advance or, say, some time period after they've installed a unit(s), they likely wouldn't account for that as a sale until the check was actually deposited.
However, like you, I'm guessing here. And what that means in the case of the two sales that have apparently happened, but have not been PRd, is anyone's guess. My guess is that they have not yet been paid, but they are holding Purchase Orders. Of course, maybe someone more in the know has a better guess.
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