Using the word "Rare" is quite common in the hobby. Most items I see listed as rare can be found quite easily. At the End of 1939 Allach had 100 standard catalog pieces, another 40 or so presentations with ease, and then there were the Ceramic pieces. 

On top of those figures were 10 porcelain vase piece's in the "500" series less than 10% of Production
The "K Series" had 20 models or just over 10% Production
The "K 500 Series" Had 12 pieces listed of which only 6 models were in prodution. Roughly 5 percent. 

Most all of these pieces were decorative or award type items that one would display, utility type pieces are just not out there. They just did not survive or were used or broken. Allach Porcelain manufactured at least 4 variant Ashtrays or "Aschenbecher" the first produced in 36/37 and discontinued and followed by a run of another model where a total of 480 were produced. A third model showing no production for 1938 or 1939. And then there is K-512. 

K-512 is the last piece listed in the production figures for 1938/39 with 219 produced in 1938 and none produced in 1939. Rare?. I have never seen an "Allach  Aschenbecher" in the market place. This is a really nice piece for the Allach collector. Guaranteed to please!.

Allach Porcelain: Ceramic K-512 Aschenbecher/ Ashtray