Step 1: Let initial coins be \( 17k \) (A) and \( 7k \) (B). After moving 108 coins from A to B: \( \frac{17k-108}{7k+108}=\frac{37}{20} \), which gives \( k=76 \).
Step 2: So A \( =1184 \), B \( =640 \) right after the first shift, with total coins \( =1824 \) fixed forever.
Step 3: For a \( 1:1 \) ratio, each box must hold \( 1824/2 = 912 \) coins. A currently holds 1184, which is 272 more than 912.
\[ \boxed{272 \text{ coins}} \]