Found new owner
The pipe from a pre WWII collection. William Demuth Company was one of the most famous and respected pipe makers in USA established back in the 1862. The first big success and fame were achieved with the Presidential series (29 carved meerschaum likenesses of American Presidents). The Royal Demuth series was developed on the very beginning of the XX century but filtered models become available only in the 1930s. Experts say WDC patented filters were quite similar to well known today Dr. Grabow and Medico paper filters. Also the pipe can easily accomodate both 6 mm balsa (like Savinelli) and European 6 mm activated charcoal filters. And of course it can be smoked without a filter as well.
The pipe markings are "WDC", "Royal \ Demuth", "IMPORTED \ BRIAR ROOT", "18", "FILTER PAT'D \ 1934". The length is 12.4 cm (4.88"), bowl's height is 4.3 cm (1.69"). External diameter of the bowl is 3.3 cm (1.3"), internal diameter is 1.9 cm (0.75"). The depth is 3.6 cm (1.42") and this pipe weights 28 gr. Briar, vulcanite stem, filter 6 mm.