To my eye the two samples you have pointed out seem to read "Un" for unknown.
I suggest you look for other instances on the surrounding pages, and then look in other record groups for those individuals to see if you can determine their birth months.
Remember that in 1900 we do not have any information about who gave the information to the census taker. See Claire Prechtel-Kluskens's article Who Talked to the Census Taker? (NGS NewsMagazine,
October/November/December 2005) for more background on how the census was taken, and the enumerator's instructions for the 1900 Census.
This is what they were supposed to fill out:
Enter in the first division of column 7 the name or abbreviation of the month in which the persons was born, thus: Jan., Feb., Mar.,
Apr., May, June, July, Aug., Sept., Oct., Nov., or Dec.
Endeavor to ascertain in each case the month and year of birth called for in column 7, but where this is impossible get as nearly as
possible the exact years of age. An answer given in round numbers,
such as "about 30," "about 45," is likely to be wrong. In such cases
endeavor to get the exact age.
The enumerators didn't always follow the instructions, but knowing what they were supposed to do gives you a baseline for evaluating the edge cases.
Make a table for all the months, and compare the different versions to get a feeling for the range of variation in the handwriting. Also, don't forget to compare your mystery letter sequence to other words on the page.