Isn't the "North Buttress" route AKA North Ridge? This is the line between Mythical Kings and The Vigil. SuperTopo has it rated at IV 5.10 A0... is that the same line? (No FA info on STopo to verify)
bird has it right in the new zion guide its 5.10 A0 great route, long with many 5.9 cruxes, the final pitch is 5.10 w/A0 bolts, it's a ful value zion climb.
The West face of the Watchman - Known Routes 1 – Mythical Kings & Iguanas – III 5.10 (Brian Smoot & Bill Robins, 1985) 2 – North Buttress – III 5.8 (Peter Avenali, Doug Tompkins & Joel Coqueugniot, 1971) 3 – The Vigil – IV 5.11 (Dave Jones & Conrad Anker, 1992) 4 – Central Pillar Direct – V 5.10 C1 ( Eric Draper & Bryan Bird, 1999) 4a- Pie For Dinner – III 5.10 (Scott Fisher & partner, late 70’s) 5 – Lowe/Weis – IV 5.10 A1 (Jeff Lowe & Mike Weis, 1974) 6 - S & M – III 5.10+ A0 (Stacy & Mark Austin, 1984) 7 – Ivory Coast – III 5.12 (Lynn Wheeler & Partner, 1995) 8 – Silmaril – V 5.10 A1 (Dave Jones & Gary Gray, 1983) 8a – Silmaril free var. – V 5.12 (Mike Andersen with Brian Smoot, 2007) 9 – Cradle to Grave – V 5.11 C2 ( Dave Jones & Brian Smoot, 2004) 10 – Strikes & Gutters – IV+ 5.10 C1 Bryan Bird & Partner, 2007) 11 – Hue & Cry - IV+ 5.11 (Dave Jones & Glen Rink, 1995) 12 – Negative Ghost Rider - IV 5.10+ C1 (Bryan Bird & Joe French, 2005)