Interesting button!
The date of 1789 corresponds to the founding of Georgetown College (now University), rather than any reference on the actual button time period. Three-piece staff type buttons of this type were primarily seen only in production beginning from the 1830's onward.
The listing in Alphaeus Albert's button reference, for Georgetown College Cadets, SU 123, only lists the one backmark of; JACOB. GMINDER. / BALTIMORE.
German immigrant Jacob Gminder was operating in the military supply business in Baltimore, from 1859 up to about 1900. According to Warren Tice in
Uniform Buttons of the United States 1776-1865, the precise wording of JACOB. GMINDER. / BALTIMORE, is listed as being a correct period backmark for American Civil War buttons.
The predominance though, of all various Gminder backmarked buttons, fall into the post-Civil War time frame, and display different variations of backmark wording and lettering. Of note here, is that fact that I could not find any reference to a Georgetown College Cadets button in Tice's expanded guide for uniform buttons, although there are many other Southern schools, colleges, and universities listed that are known to represent pre-war and war time period buttons. Considering the guidebook by Tice only covers the span up to 1865, lends the possibility that this particular Georgetown College Cadets button is considered post-Civil War by leading collectors.
CC Hunter