A dumb statement in last post "you can forget about Frank and Mable": If he is not your Frank then you have another Frank to trip over so you can't forget him.I am in too much of a rush and don't have the time to give you the help you may need.
It would be great if you found a Frank Monty Taylor in a record at ME Archives or Portland City Clerk.The problem is he does not show up (I don't think) on 1900, 1910, or 1920.And the birth record could be a "looks like".
The most likely thing is that when he bacame an adult he reverted to his birth name and stopped using what he was called.Less likely he went the other way. The problem for you is that everything hangs on a birth record, likely to be one (I don't think Monty is a likely middle name) and a question of exactly where and possibly when he was born.
Get as many GOOD records as you can.That means the real ones NOT JUST OFF THE INTERNET. If push really come to shove go to Deeds.Deeds are perhaps the best record for jumping time and distance.