係呢到back字點解??

若你查考Cambridge Dictionary，在back條下，有一段是這樣解釋
back (RETURN)
adverb
1 in, into or towards a previous place or condition, or an earlier time:

back在這裏解作”返”，”回到”之前的地方、狀況，或（指時間）以前，上溯等
如：back in Middle Ages(上溯到中世紀）
　　The last time we saw Lowell was back (= at an earlier time) in January.（上次我見Lowell的時候，要追溯到一月）
　　This tradition dates back to (= to the earlier time of) the 16th century.（這傳統要追溯到十六世紀）


你的示句”back in those days”要追溯到那那日子（但更好譯作：在那些日子以前）

which was undoubtedly unusual back in those days

back in those days
解 "o係果個年代, 果個時候"

which was undoubtedly unusual
解
undoubtedly (毫無疑問地, 肯定)
unusual (旱見, 唔正常)

成句即係
o係果個年代/果段時候, 佢獨力撫養六個仔女肯定係好旱見!

返黎的意思