Hungarian `Postage Due` (PORTO) stamps 1903-1945.First Hungarian `Postage due` was issued in 1903 (for a comparison- France 1859 / Italy 1863/ Romania 1884/ Austria(!) 1894). Of course the postage due was applied, but marked with handwriting, different handstamps etc.
Sorry for the quality of some stamps, but right now I have only these to scan.

1903/1905/1908/1909/1914 issues- same design, different perforations -green background, BLACK numeral (as the first stamp 1/1)
1915 - new value, red overprint - `20` on 100 filler
1915 - same design, RED numeral (first row, stamp 2)
!! 1918! ANSWER TO THE QUESTION!! - as stocks of the `Postage due` stamps ran low (due to the constantly changing postal rate) Hungarian Post issued an order at 15.06.1915 allowing usage of postage stamps as `PORTO`. These stamps were marked with capital letters of `T` or `P` or with the inscription `PORTO`. Many post offices handstamped or marked postage stamps in advance. Different kind of markings were used, but these stamps (as presented by Prahanoaki or first row 3 stamp) are not considered local overprint, as thee were officially ordered by Hungarian Post. The order was short-lived, by mid-november 1918 these `auxiliary` Porto-stamps were prohibited. ONLY MENTIONED IN `HUNGARIAN STAMP CATALOGUE` , BUT NOT LISTED.
1919 - `KÖZTÁRSASÁG` (Republic) ovpt. First row stamp 4
1920 - same design, again w. BLACK numeral
1920 - `TANÁCSKÖZTÁRSASÁG` red oveprint oveprinted with black weat ear -as on this postage stamp:

1921-1925 red overprint (PORTO & new value) on `Harvester` set, second row stamps 1-2
1926 - new design, second row stamps 3-4
1927 - overprint on 1924 `Icarus` stamps, second row stamp 5
1931 - black ovpt. on 1926 postage due, third row stamp 1
1934 - new design blue, 3 row stamp 2
1941 - new design brown, 3 row stamp 3
1945 - previous overprinted, 3 row stamp 4