Upon withdrawal for exportation of metal from a bonded warehouse engaged in refining, or smelting and refining, part of which metal was obtained from imported crude metal and part from crude metal produced by smelting imported materials, the warehouse account shall be credited with the quantity of metal so withdrawn, plus (a) the refining wastage allowance prescribed for that establishment, and (b) the smelting wastage allowance prescribed for the establishment in which the imported materials were smelted, and (c) any dutiable metals shown on the warehouse entry or the rewarehouse entry filed at the first-mentioned warehouse which have been lost and are attributable to the exported product. However, upon withdrawal of such refined metal for consumption, no allowance shall be made for wastage except where the withdrawal is made from a bonded Customs warehouse other than a bonded smelting and refining warehouse.