As you may know - this is a "quality mark" and not a "back mark" so to speak. I do not know who may have been the actual "maker" of the button.
The "quality mark" generally cannot be attributed to any one button maker but was a popular way to grade buttons by both British and American button makers from 1790s through the early to mid 1800s sometime.
I agree with Muddyhandz that this one around 100 years old based on construction.
Someone with a button book may be able to help a little more.
Quality Marks like "double gilt", "extra orange", "extra rich", etc. simply indicate that it's a super sexy and heavily gilt-ed button... it's a sales pitch more than anything.
Your button, I believe, is supposed to say: "
non * plus * ultra" [
ne (non) plus ultra] which in Latin/Greek means: Nothing further; the uttermost point;
perfection... It's a indicator of quality! I think however the maker screwed something up and replaced the "S" with a "5" and made some error buttons.
Tell yer buddy congrats... I think it's a beauty.