Some information on the following 4 pieces (from top to bottom and from left to right):

1. 28th April 1891 - from a location in
Estlyandskaya province (undecipherable / probably in present-day Estonia) to a doctor in the Wiedicon district of Zurich, Switzerland. Franked correctly at 10k.
2. October 1879 - Postal stationery card posted on board a (railway)
Travelling Post Office (TPO) to Warsaw (arrival postmark of 20/10/1879). Two interesting postmarks:
a) 3-line date TPO postmark with inscription "Pochtoviy vagon No
?" (==postal wagon no).
b) special round mark, used in Warsaw to indicate time of delivery of correspondence: "8 ch(asov) / vech(era)" = 8 o'clock / evening.
3. Picture postcard with view of Riga, present-day Latvia. No posting details provided.
4. 4th March 1935 - Registered cover from
Leningrad (3rd district sub-office) to Melbourne, Australia. Standard international "R" label. The violet "RECOMMANDE" mark is of private origin. Franked correctly at 35k.
Nothing out of the ordinary in these pieces, except maybe the round mark of Warsaw. The TPO number should be visible if you look closer.