You should study the pictures of the carbines.  No, you do not have to have the upper guard for the gun to work.  I have several that are missing their hand guards.

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Carbines had a spring to hold the band on to the barrel.  Rifles had a brass pin. The rifle band will hold without the upper guard because it is a clamp affair if I recall.

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Also look at the pics of the hand guards.  You will see the screw holes are standardized but the sights come in "forward" designs like the 1901 or "rearward" like the 1898s and 1902s or a "neutral" such as the 92 and 96 that are the same length as the base.

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