Frank would be a better canidate for service in WWI, his death certificate could be mistyped. The only military record I find for Carl is a WWI draft registration card record. Frank was of an age where he should have filed one too if he wasn't already enlisted. So not finding one would send me looking at WWI for military service. Also the NARA fire only destroyed Army/Air Force records so if he happened to join another service his records would be available. And only 80% were burned so you might get lucky still so try for both WWs. Frank is single living in his parents household in 1920 census. In the 1930 he is married but the Veterans question column is left blank. For others on the page there are more blanks and yeses and noes so it's a guess. Also look at the local newspapers of the time they usually reported on the local men as to their enlistments, who left for basic, who's home on leave, promotions etc.