(mobile billboards constitute tangible personal property that qualify for I.R.C. Sec. 199 deduction, but traditional and modern billboards are real property (inherently permanent structure) not qualified for I.R.C. Sec. 199; under facts of CCA, taxpayer installed traditional and modern billboards on leased land with leases ranging from 30 days to 20 years and leases continued until billboards no longer profitable; mobile billboards relocated on frequent basis; mobile billboard attached to truck that is moved to various locations; traditional billboard attached to wooden pole; modern billboard attached to steel frame attached to steel poles; mobile billboard not attached to real property and not intended to remain stationary indefinitely; traditional billboard an inherently permanent structure; modern billboard inherently permanent structure).