If you are on Insider and you use the button push method (hard rest) or software method (factory reset) you get back to the very same version you have now.
If you use WDRT you get an earlier version. The last official. Note that if your device was made for W10M you likely get 10.0.10586.67 or .127. If your device was NOT built for W10M, you get the last official 8.1 for your device.
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