Pneumatic mail with postage dues

Up to 31.7.1911, un- or partially-franked mail found in a red (pneumatic) box was handled as "surface mail" and delivered as such. Postage Dues were levied if appropriate. Pneumatic mail up to that date had to be fully stamped, and does not occur with Postage Dues. Then, however, ordinance RGB 127/1911 of the k.k. Department of Commerce (dated 28.6.1911 & republished as usual as PuTVBl 91/1911) announced that from 1.8.1911, insufficiently stamped pneumatic mail would be accepted, and charged double the missing amount, which the addressee would pay. Completely unstamped mail, however, continued to be handled as surface mail. Pneumatic mail from 1.8.1911 was as a rule fully stamped; "postage-due" items are infrequent and rarely found. However here is one:

The date is 14 Feb 1912. It is a small letter, 12 cm x 9.4 cm, addressed to a collector, but not a philatelic item. It is franked with two 10h 1908 stamps, and is cancelled with the Rohrpost cancel 1/1 Wien 15 *P*. Under the stamps is a blue crayon marking 50h, upper left a blue crayon 62 crossed out, and underneath a blue crayon 57 [this will be the Instradirungs-chiffre]. On the back are two 20h and one 10h 1908 postage dues with two cancels 6 Wien 57 *P*. There are two vertical folds through the envelope, which is fairly thick paper. The original franking of 20h is correct for a 21-250 gram ordinary-mail letter; but the weight limit for pneumatic mail was 20 gram. The letter was probably dropped by mistake into a Rohrpost collecting box or slot, and since it was partially franked it was handled as the regulation said. The pneumatic rate was 45 heller, the missing postage was 25 heller, twice it is 50 heller. Perhaps the sender was unsure of the weight, and stuck 20h on just to be safe – then dropped it in the wrong box!

The second example was sent express from Graz to Wien on 14 Feb 1916. Note the "Express zu zustellen" (deliver by express) written on the front in a different hand from the address. The letter rate was 10h, the express fee 30h, so it was 10h underfranked. Graz cancel timed 10am. Graz didn’t levy postage due; maybe they noted the lack of an Express label and treated it as an overfranked letter. Somewhere along the route the cover received 20h of postage dues, calculated as "double the deficiency". These are cancelled WIEN 53, possibly when Herr Taborsky paid up. The item has been treated in Vienna as an underfranked pneumatic item, so it was entitled to be pneumatically forwarded and taxed at twice the deficiency [10H letter + 30H pneu = 40H, franked 30H, deficiency 10H, due 20H].

From 1.10.1916 to 1918, a partially-franked item would be forwarded by pneumatic post provided it carried at least the pneumatic surcharge (1916 Postordnung §98); any deficiency was (according to §48 or §50) doubled then rounded up to the next 5 above, and the amount which the recipient had to pay indicated with Postage Dues. If an item didn’t have enough to cover the surcharge it was treated as underpaid surface mail. The second (1922) issue of the Postordnung reversed this change to §98: only fully franked items would be forwarded by pneumatic post.