The M14 rifle was first furnished with a walnut stock, then with birch and finally with a synthetic (fiberglass) stock, which was adopted for use in damp jungle environments in Vietnam, since the wood versions would often become warped and swollen with moisture. The stock was also fitted with a hinged shoulder rest for improved user comfort when firing from a prone position. Original equipment walnut and birch stocks carry the Department of Defense acceptance stamp or cartouche (an arc of three stars above a spread-winged eagle). These stocks also carried a proof stamp, a P within a circle, applied after successful test-firing.