A projecting part of an object by means of which it is secured to a handle, or to some other part; anything resembling a tongue in form or position. Specifically:

(a) The part of a knife, fork, file, or other small instrument, which is inserted into the handle.

(b) The projecting part of the breech of a musket barrel, by which the barrel is secured to the stock.

(c) The part of a sword blade to which the handle is fastened.

(d) The tongue of a buckle. [Prov. Eng.]

Source: Webster’s Revised Unabridged Dictionary (1913)

Duo-tang is a brand name & most likely refers to the two, bendable, metal clasps that are used to secure paper in the folders.

It also has a Chinese origin:

Tang \Tang\ (t[aum]ng), n. [Chin. T’ang.]

A dynasty in Chinese history, from a. d. 618 to 905, distinguished by the founding of the Imperial Academy (the Hanlin), by the invention of printing, and as marking a golden age of literature.

Source: Webster’s Revised Unabridged Dictionary (1913)