Edited text from one of the owners:
Firstly, despite being based on the Swiss K31 rifle, that's where the similarity ends. These rifles were never made for the Swiss military.There were two distinct orders placed by the Nicaraguan Government in 1948. This was confirmed by the Hämmerli factory. The first batch (FK001 to FK100) were fitted with diopters, completely different in design to what we see in use on K31's today.
There was no provision for a military-style ramp sight. A tunnel foresight, which was a permanent fixture, took different-sized metal ring or blade inserts of various sizes. The top hand-guard was of a one-piece construction that went right to the start of the action, similar to the British No.4 top hand-guard. The Nicaraguan rifle team used these rifles in competition, and also the Pan-American Games.
The second 100 rifles were the sniper rifles as referred to in the above postings. Serial No's were ZK101 to ZK200. The side-mount was a wonder in itself; every mount required individual hand-fitting. the attachment operated on a lever camming principal. Immediately hidden behind those two bolts on the LHS of the receiver were two vertical slots, to which a lever on the mount, when placed over the two bolts at the 12 and 6 o'clock position, was then rotated to the 2 and 8 o'clock position, which locked the mount to the rifle. Once the mount was individually fitted, it was then numbered to the rifle.
It would be a miracle if a side mount off rifle ''A'' would fit onto rifle ''B''; that's how exact those tolerances were. There were two versions of these mounts, the most common being the pattern that placed the scope offset to the left, so that the six-round charger could be used. The other mount was designed to place the scope directly over the centre of the action, thus prohibiting the use of the charger when the scope was attached.
The scope used was a Kahles-Wein ''H 4x60'' single-turret for distance adjustment (0 -10) only. Wind adjustment was set by loosening and tightening two opposing screws in the front mount-base. Any adjustment required meant that the mount had to be removed completely off the rifle. Once the rifle was zeroed in, the wind adjustment was not meant to be touched in the field. Focus was fixed at infinity, but the reticle focussing was achieved by moving one's thumb sideways either way on a radial-shaped platform forward of the eyepiece.
Hope the above helps.
OldTimer
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The tunnel fore sight and diopter on the diopter equipped rifles were Hämmerli design also used in their K31 based target rifles.