If the uncles were already serving in the National Army or the Texas National Guard at the time of the WWI Registration then they will not have cards, as those men were exempt from filing. Junius was married and claimed a bum ear so he did not Volunteer before Cards had to be filed.
Have you tried the Texas Archives? Most if not all Texans served in either the 36th Division or the 90th Division. There was a 95th but no information has been confirmed as actually being assigned men. It may have been organized on paper but maybe the war ended before it was more than on paper.
Some sites, if you don't already know about them:
http://freepages.military.rootsweb.ancestry.com/~worldwarone...http://www.txgenweb.org/tx/txlinks.htmlhttp://www.texasescapes.com/WorldWarI/WWIChronicles.htmBoth Uncles state they served in WWI on the 1930 Census question asking.
By the way they will not have WWII Draft Registration Cards as those only needed to be filed by men born between April 28, 1877 to February 16, 1897. Tyler was born May 1897 and Paul Sept 1899 according to 1900 census.
LDS Family search has an image of Paul Thomas Darwin's death certificate and it states he was a Veteran of "US Armed Forces" in WWI.
John T Darwin died in CA and they only have an Index. Since the Index has a blank place of birth looks like the informant may not have known of Military service either to contact the VA for Burial, also has mother's maiden name as Halch.
Usually the VA BIRLS file is of Veterans whose family files for a VA benefit or National Cemetery burial.
Having a WWI Draft Registration Card does not mean any man was conscripted into service. Only some were and others still volunteered after filing.