What background information did Watson and Crick have available for developing a model of $DNA$? What was their contribution?

(N/A) Watson and Crick utilized the following information to develop the double-helical model of $DNA$:
$(i)$ Chargaff's rules,which state that the ratios of adenine to thymine and guanine to cytosine are equal ($A=T$ and $C=G$).
$(ii)$ $X$-ray diffraction data produced by Maurice Wilkins and Rosalind Franklin,which provided insights into the physical structure of $DNA$.
$(iii)$ Their contribution included: $(a)$ Proposing the double-helical structure where complementary bases pair via hydrogen bonds,$(b)$ Suggesting a mechanism for semi-conservative replication,and $(c)$ Proposing that mutations could occur through tautomeric shifts in nitrogenous bases.