I've got the same .plist (which I obviously didn't manage to find) with the same data string in Sierra, and I suspect that you've gone me one better and hit on the real answer, but I can't check, because I haven't got any updates waiting at the App Store.
At any rate, I'm happy to have pointed you in the right direction, and I'll guess that your problem's resolved for good as long as "Check Now" ISN'T checked in System Prefs > App Store.
.Plists open in both TextWrangler and TextEdit, but the latter doesn't show you a readily editable format.
(I thought of suggesting that you edit the .plist I found with TextWrangler, but I didn't know if you had it and took the easy way out.
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