If it's of any help, a Wheeler was someone who repaired and maintained the gun carriages, limbers etc. The civilian equivalent was a wheelwright. Therefore I would have expected Staff Sergeant Harper to possibly have had a civilian trade prior to joining the army which had similar skills, such as a carpenter, coach builder or wheelwright. It's highly unlikely that he would have had a completely unrelated trade like a baker or shoemaker. Since he had reached the rank of staff sergeant it implies he had been in the Army for some time, say a minimum of 8 -10 years. Possibly he was a career soldier and that's where he learned his trade of wheeler.