The first cover is from WIEN 101 - a 30 heller Rohrpost envelope addressed to Switzerland, with no visible return address. The Rohrpost imprinted stamp is cancelled 8 50V on April 29, 1900, and is crossed out in blue. There is also a 25 heller stamp affixed and tied with a Striche cancel on 30 April 1900 at 15/1 Wien 101 3-4N. Note the different dates!
On the back is a message "Im Sammelkasten offen vorgefunden. Inhalt briefamtlich verwahrt" signed by TWO signatures. It is sealed by a big paper seal K.K.Post & Telegrafenamt Wien 15/1 101, tied to the envelope by two strikes of a boxed WIEN 101 marking. This looks like the marking which a pneumatic office would use to 'certify' a message and the sealing of a letter. There is also a Wien 104 Rohrpost cancel on the back at 9:40 V of 29 April. There is another Striche cancel on the back from 29.4 but the post office is illegible.
What might have happened is that the letter, using a Rohrpost envelope, was found in a red pneumatic letter box, collected thence, and taken to Wien 101. They found it open, and also decided that the imprint was inadequate or invalid for international postage. They found inside who had sent it, & sent it back for a 25 heller stamp. That was put on, it was remailed in normal mail the next day at Wien 101, and hopefully arrived in Bern soon afterwards. If it hadn't been assumed to be Rohrpost because of its envelope, would the 30 heller stamp have been considered as paying for international mail? This has been seen on other examples; perhaps in this case the posting in a red letter box plus the pneumatic header not being crossed out led to its disqualification.
Next is a lettercard posted in 1901 and handled differently: it received a special explanatory boxed cancel "Aus dem Briefkasten / Postamt Wien 1 am {date & time}". That card was actually sent to London, for which the rate was 25H. The handwriting looks English - perhaps the sender hadn’t realised that red post-boxes in Austria weren't the same as those in London!
