Interesting ... Don't see why it couldn't be an M3. If you note it goes from M2 to M4 for known models. No one knew about 633, nor blind back, etc. Just like the 613 ad Lance says he has from Niagara Falls collection.
Perhaps yet another model we can add to the growing info on this Co.
If you click on view all it will open the album with all 4 pictures. You can then click on the individual pictures and enlarge them.
When I enlarge the data plate it appears that it was stamped M-3 and then over stamped with a 4. The serial number is very low at 1689, so maybe M-3 was going to be the next model after M-2. Or some employee thought that.
As always in the world of Durant "never say never" !!!