@phxrock
These began manufacture in the 30's (as evidenced by the patent date) and I haven't seen much evidence that these were manufactured much past the 40's. These enjoyed popularity around the same time as the hesson guard series and were WDCs most popular filter pipes of that era. That's based solely on the advertisements I've seen and could be totally wrong, but I'm pretty sure that's the proper time frame. That being said, it appears that a bunch were sold as they aren't particularly rare. You can always find several for auction on ebay at any given time. They usually run in the $10 to $20 dollar range.
I have several and find them to be great little pipes. They ARE a filter pipe though, so those that don't like filters might not enjoy the royal demuth line from WDC. If you don't mind filters then these are fine pipes as they come from the earlier days of WDC and are made from quality briar, unlike the later lines of WDC pipes where SM Frank really degraded the line with cheap briar, poor manufacturing techniques, and nylon stems. Those earlier lines, while mass produced, still showed good workmanship and were made from quality materials.
Just don't over pay for it as royal demuth's are readily available and shouldn't command a premium unless unsmoked. Antique stores have a tendency to overprice old mass produced pipes in my experience. I might go a few bucks more for the style that has the silver rim cap, but still wouldn't pay much over $20 for one. There are just too many still available on ebay for a reasonable price.
Hope that helps.