Sections 191.001 and 191.002 do not apply to:
(1) a structure that had been built or was in the course of construction on June 18, 1925, or for the building of which material had been purchased on that date as provided by a prior contract or plan; or
(2) material that had been placed on June 18, 1925, or purchased for placing on that date as provided by a prior contract or plan.