The Roman number LI corresponds to the arabic number 51.
There is no common alternative notation for this number.
You are looking for acronyms beginning with LI? Here is our list with acronyms starting with LI.
Interesting Fact: In the French Republican Calendar, initiated during the French Revolution, years were numbered by Roman numerals – from the year I (1792) when this calendar was introduced to the year XIV (1805) when it was abandoned.