The eyelets, present on all variants of the "1951 Models" and up to variant 5 of the "1961 Models" were designed so the knife could be used as a plumb-bob by introducing a string.
The Swiss Army requested this modification, to determine the aiming angle when using the assault rifle "FASS 57" as a grenade launcher.
When the Swiss Army adopted the new FASS 90 rifle the eyelet became redundant, it was abandoned on variants 6 and 7 of the "1961 Models".
(*) Source : www.couteauxarmee.e-monsite.com