Forgive my ignorance. But when refering to a savage 99 what does pre mil mean?

A rifle with a serial number under the one million mark is said to be a "pre-mil". All of these have serial numbers with no letters as a prefix. Any rifle with a letter prefixed number is a post mil.

Pre million Savage 99s have square barrel threads and a smaller shank than later Model 99s. The later model 99s have v threads. The later models (except for the EG) also have the tang push button safety.