Here's the info on the +1

(Referring to a 7x57 barrel marked, "-1"), "That mark is a barrel fitting notation and the absence of a "+1" on the underside of the barrel was evidence that the barrel was not original".

In Mannlicher-Schoenauer, Evolution of a sporting rifle, by Don L. Henry. Australian Shooters Journal, Nov. 1991, pgg 55-59; Dec., 1991 pgg 57-59 & pg 71.