Dahl Master Slide Interlocking Rule Interlox

This is an interesting type of boxwood rule, featuring the sliding extensions, instead of the more common zig zag folding type rulers. The slides are linked by brass guides and spring clips that hold the rule opened, or release the rule with a slight finger presure on the top of the clip. The one is marked "MASTER SLIDE RULE INTERLOX DAHL MFG. CO. N.Y.C. MADE IN U.S.A.". There is a patent mark also, "PAT'D 12/13/1910 PAT'D 7/29/1913". It also has inside, and outside measurement sides of graduation. The side marked "INSIDE" is confusing to me, as it startS at the left with a section graduated from 1-8" then the next section from the left is marked from 72" down to 9". I have no idea how to use that side. On the side marked "OUTSIDE" the graduations are from the left 0-72". This rule is in good working condition, all the brass locks hold well, a few sections have some minor warping on the wood sections, we note one hairline crack on one side of the wood from 1-1.5" are under the INTERLOX logo, it is solid as it has been repaired with glue, a few tiny edge dings noted. The graduation markings are quite good, a few minor stained areas, and a few areas where the markings are a bit lighter than most of the rule, but certainly useable. The brass hardware is all in great shape, no damage, light tarnishing. One of the benefits to this type of rule versius that folding zig zag rule, is this type will close from full opened to completely closed in just a few seconds, much faster than another one could close a zig zag rule. Overall, a nice collectible rule from the early 1900's era!