Alexander Sturm / Guilloche locket part

Another one flea market find. Guilloche / enamel, the metal was acid-tested as silver.
Watch case part, or part of a locket pendant? I would say this was originally part of a watch case, but I am not sure and I would like to know your opinion on this.
What about the manufacturer? As you can see one hallmark is a shamrock, but I was unable to find such a maker.
I would be very thankful for your help, as usual!

P.S. Identified by IVAN49 as a work of Alexander Sturm.
From internet:
Our story begins in 1882: Alexander Sturm, a trained gold- and silversmith, fulfilled his dream of manufacturing his own silverware.

The Vienna Ringstrasse culture was at its zenith, and the dominant style was Historicism. The needs to represent by the upper middle-classes were satisfied by exquisite silverware. The workshops produced products that were virtuoso feats of craftsmanship of incredible stylistic diversity.

In a unique coexistence between tradition and modernity, Vienna establishes itself as the intellectual centre of Europe around 1900.

Also the Manufacture was enthusiastic about the ideas of the Wiener Werkstätte.The most important customers belonged to the intellectual elite, including families like Wittgenstein, Kohn and Mautner Markhof. It is the heyday of the silverware production.

When the Wiener Werkstätte closed its gates in 1932, as the Second World War took its toll, the successors didn´t give up. In the 1950s and 60s connections were once again established with the art world avant-garde. Designs like those from the Werkbund architect Oswald Haerdtl help the Viennese silver tradition to reach new heights.

However, despite this, the year 1988 marked the end of the firm Alexander Sturm. It had lasted for over a hundred years.