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Same scenario in Fall River where I did mine, EEA and DOR were right across the hall. I understand that's true at the Boston office as well.
It is absolutely absurd what it takes to get your reg done as a non-resident. There aren't even instructions for it on the EEA website for OHV registration, I've spent more than an hour making phone calls and sending emails to figure out what they should do. You would think it would benefit the state to make that process as simple as possible.