In the previous paper [15], we determined the structure of the Galois groups $\text{Gal}(K_{ur}/K)$ of the maximal unramified extensions $K_{ur}$ of imaginary quadratic number fields $K$ of conductors $\leqq 1000$ under the Generalized Riemann Hypothesis (GRH) except for 23 fields (these are of conductors $\geqq 723$) and give a table of $\text{Gal}(K_{ur}/K)$. We update the table (under GRH). For 19 exceptional fields $K$ of them, we determine $\text{Gal}(K_{ur}/K)$. In particular, for $K=𝐐\left(\sqrt{-856}\right)$, we obtain $\text{Gal}(K_{ur}/K)\cong \widetilde{S_4}\times C_5\text{and}{K}_{ur}=K_4$, the fourth Hilbert class field of $K$. This is the first example of a number field whose...