An original M1956 field pack, generally sound condition, no rips or tears, appears to bear a faded date stamp of 1959 as well as E-60, suggesting the contract was placed in FY1960. There seems to be no consensus on actual contract/manufacture dates for these packs that bear two dates. The 1959 may be the date of the specification.
This is the first pattern, not the later M-1961 pattern, as this one has two canvas flaps across the top and lacks the rubberised throat/lining.