The 571-5 means that it is a Model 571 with a 5 inch blade. A hunting knife model from the 30s, if I remember correctly. The 5 in 571 means "stacked leather washer handle". So 571-5 breaks down to "leather handled, 5 inch blade, pattern 71.
The original handle would have been leather washers similar to those on other leather handled Kabars. That means that the current handle is a "repair or remodel", most likely due to a damaged/destroyed/rotted handle. The liberal use of what appears to be acrylic of some sort, this "might" be a "theater knife", a privately purchased knife used by a member of the armed forces during WW2. Then again, it could just be someone found a 571-5 with a bad handle and replaced it in "theater knife fashion" any time since the knife was made to make it look like a theater knife.
Without any provenance on knife purchase and ownership, one can't say when the repair/modification occurred. The sheath appears to be old enough to be original, but appearances can be deceiving. Any environment to which the knife was exposed that caused the handle to deteriorate to the point of requiring replacement would have killed the sheath even faster than the handle.