I contacted Mike Mahler for any information he might be able to provide. His initial reply:
My intuition is that it's an Eastern/Midwestern cert copying the primitive style of the Western certs of 1863-4. Note the power of atty is to a guy in Cook County IL. I have seen one "pseudo-Western" cert out of NY, the Gasherie if memory serves, but never a "Montania" like this. Very interesting!
I then replied with the information that Bart provided above, to which he responded:
Dan, Very interesting, thanks, first I've heard this term. However Montana was created in 1864 and everyone out there knew its name. This company formed 1866!
Hmm, hold on there are some MONTANO postmarks. But no Montania.
Here's the Gasherie. Clearly pseudo-Western!