Three train seat reservations used on the Grand Tour. The Sinclairs must have kept their reservation slips.
Then (and now) when travelling by train in Germany you needed a train ticket of which the fare is calculated by the distance, and if you wanted to also be guaranteed a seat, you need a reservation. It's still like that, though a reservation is free with 1st class tickets, but they are not compulsory.
The ticket says on the front "Platzkarte" which is best translated as "seat reservation," and mentions that it is only valid if there is a stamp on the back. On the back it says that it is only valid in combination with a valid ticket, that you need to occupy your seat before the train leaves, that you must leave something indicating the seat is occupied if you leave it, and that there are no refunds. It's basically the same today, only reservations look very different now.
It would be interesting to find out what train they were on, and what route it took. In 1926 the railway network in Germany was very different from now. The numbering is also interesting. Nowadays you will not find seats 29 or 30 anywhere on a German train.