It is indeed the Neuchatel assay mark (referred to as the "chevron"), which represents 18 carat gold (and I believe .925 silver). It simply indicates that the case was assayed in Neuchatel rather than the Swiss national assay office (which used the head of Helvetia and the grouse marks). That in turn would suggest that the case may have been made in the canton of Neuchatel, which for example includes La Chaux de Fonds.
The Chevron was the hallmark used at Neuchâtel from the 1700's until the 1880's. The added word "argent" would distinguish the metal used as silver (obviously) in opposition to the stand-alone Chevron which would indicate 18k gold. The minimum fineness for silver from Neuchâtel was 812.5/1000 for many, many years. The Chevron hallmark was merely superceded in 1881/1882 when the Swiss Federal Government assumed control of hallmarking from the Cantons.
I didn't think that the Helvetia was introduced until the metal act of December 1880, prior to that I thought each Canton/City had various unique marks.
The Neuchatel chevron is well-known and seems to appear quite frequently, but I have never seen an assay mark for any other canton. Did others exist?
Also, I have never seen a canton letter associated with the Helvetia head ... or have I just not noticed it? Would it appear immediately under the Helvetia, or might I have mistaken a letter for a jointer's mark?