That's very interesting; I have seen the odd SM205/60 but never a 105/60 - this one is in delightful order with what looks (typically for a Mabie Todd) to be a lovely nib. People erroneously describe these as being made of black chased hard rubber, which they are not; they are made from Celluloid. The filler lever, section and feed of course are of black hard rubber.
The clip dates the pen to the mid to late 1830s; I would guess around 1936. You are fortunate to have found such a nice example; I hope it will give you plenty of writing pleasure as I sure it will..