Guide Slot Length is the distance between points A & B.  

The guide slot length is the measurement of a shallow narrow slot extending from the deeper lug slot toward the muzzle ring on top of the bayonet's hilt.  This guide slot was designed to accommodate a guide bar that had been added to the bayonet lug on the rifle. Guide bars were used to strengthen the attachment of the bayonet to the rifle.  They can be as short as 10 millimeters or run from lug slot to muzzle ring.   Many bayonets do not have a guide bar slot. 

The guide bar was introduced in the United States in 1854 with the introduction of the Model 1841 Rifle Bayonet Type II.  In 1857, the guide bar was eliminated at US arsenals with the introduction of the Model 1855 Rifle Bayonet.  A short period within federal production but the use of guide bars continued well past the US Civil War within the numerous commercial contracts.