Yes, the Postal Wagon mark is from a TPO.
The route is identified by the two numbers at the lower right, 5-6, 5 relates to the outward route and 6 for the reverse.
My only reference for these describes 5-6 as relating to the route Vilna to Warsaw from 1872 to 1913 but it's silent on dates before then.
Perhaps someone can add to this?
The sideways figure "1" in the CDS is the station number and the number in brackets at the bottom is the serial or handstamp number.
The elliptical FRANKIROVANO mark means "franked" but I don't what purpose it has here.
The top left appears to have an instruction in Russian from the sender that this is to Austria.