What's the missing character: 幼儿为「么？」?
It is a list of 四川話 (Sichuan dialect) terms, I suppose it showed other provinces' visitors some basic terms in Sichuan  dialect

It includes : 

夫妻為 「兩口子」(husband and wife is "兩口子")

最小兒女為「么兒」(smallest child is "么兒")

小妹妺為「么妺」; 「么姑娘」(youngest sister is "么妺" ; "么姑娘"

小弟弟為「么兄弟」(youngest brother is "么兄弟")

The one sentence you are asking is:


  幼叔為「么叔」(youngest uncle is "么叔" )


The  Chinese 「  mark below 为  bent left instead of bent right, because the text was written in vertical right to left Chinese style.

I bet Sichuan dialect speakers can guess the missing characters better than Mandarin speakers. I couldn't figure out 「屋里」 in 男人稱妻人為「屋里」(men call their wives as "屋里")