This Mabie Todd dip pen is made in gold-plated (or possibly gold filled) brass with mother of pearl. It holds a Mabie Todd 313 #4 flex nib.
The pen probably dates from the late 1800’s. Mabie and Todd started making pen holders in New York in the 1860’s and they started making gold nibs when the Bard Brothers joined the company in 1873.
By 1884, the company started making the Swan fountain pens in, which dates this pen to some time before that.