After rereading this, perhaps the problem is as simple as the board really has the DFU, and not the CDC bootloader. How do you know which bootloader it has, other than the vendor's statement?
Are you using Linux and not Windows (the /dev/tty.usbmodem12341 suggest this). It's actually pretty important to mention which OS and hardware you're using, Windows, MacOS, Linux (which release too), Plan9, BeOS, PC, MAC, IBM 360/44, whatever.
Could this board have shipped without any printer firmware? Has it ever worked? Have you at least gotten whatever you're using to drive it to get the usual Marlin startup messages, or anything other than recognition of the port?
makemendel.com is based in Mumbai, India (my daughters have been there

). I haven't dealt with makemendel before and am not familiar with their reputation. They do produce PCBs in India, but it's likely the board is a chinese sourced copy, and possibly whichever clone was cheapest on the day they ordered them. These range from utter crap to pretty good. The $80 they list isn't a whole lot cheaper then the official board, and that has seriously good support behind it.
From your IP address it looks like you might be in Sweden (their reverse lookup is seriously munged; it took a bit of doing to figure out what provider owned the address). That does make it a bit harder to order from Printrbot with the shipping and import taxes, don't know how that applies to online orders from India.
Kirk