That the 1918/7-S is a key to the series is never in question, since only the 1916 is more valuable in lower grades. The about-uncirculated level is where it begins to overtake even the 1916 as the premiere Standing Liberty quarter, and above this level, it becomes the most elusive coin, practically unknown with full-head details, and very expensive in all but the lowest mint-state grades. While a set that is otherwise comprised of fully choice uncirculated pieces may not be quite compatible with this AU-55 example due to strength of luster, good production and very decent preservation mean that it will assuredly fit in seamlessly with a set of high-end AU or nominal BU coins.