How does the Pre-Classical written language compare with Middle Mongolian?

The Pre-Classical period of the written Mongolian language corresponds to the Middle Mongolian era (13th–16th centuries).1

When comparing the two, the Pre-Classical written language is considered slightly more archaic than the contemporary Middle Mongolian that was recorded in Chinese transcriptions—such as the Secret History of the Mongols (c. 1240)—as well as in various texts and glosses found in Chinese, ’Phags-pa, Persian, and Latin scripts.1

While the Pre-Classical written form provides a window into the language of that time, it differs from the spoken language of the era, just as the later Classical (or Literary) Mongolian would eventually diverge from the evolving spoken dialects.21