Rex Kamstra's website
claims that this is indeed the tallest lookout that was built in the Pacific Northwest at 123 feet. This claim I believe originates from Ray Kresek's book on fire lookouts although I'm unable to find the book at the moment to verify that. In any event, it is certainly the tallest lookout remaining in the state of Washington. The large lookout gives Moses Mtn the appearance of having a large oil derrick atop the summit when it is viewed from nearby peaks like Tunk Mtn.
As a minor footnote relating to the elevation and prominence of this peak, the BM data linked to from the link at that LO website shows BM elevations of 6778, 6778 and 6775 so maybe a few feet higher than the 6774 on the map. The BM is set in an obvious rock which appears to be the HP.