It's like this:
=LET(reg,VSTACK(start:finish!K2:K54050),
date,VSTACK(start:finish!M2:M54050),
sum_insured,VSTACK(start:finish!U2:U54050),
DROP(REDUCE("",TOCOL(A3:A54050,1),LAMBDA(x,y,
VSTACK(x,TAKE(FILTER(HSTACK(TEXT(date,"Mmmyyyy"),sum_insured),reg=y,"Not Found"),1)))),1))
What is hard about...