Unused stamps come in two flavors. Mint and Mint, never hinged (MNH). Material is so readly available that collecting unused examples without gum is rather pointless. For those favoring MNH material, be prepaired to face some high asking prices. The never hinged material is scarce for a number of the values in the period. For over 60 years, mint sheets have been torn up to manufacture "packets". For the most part, this "packet" material has been hinged by beginning collectors. A good rule of thumb is that any stamp that has a catalog price above the catalog's minimum is going to be hard to find in MNH condition. The stamps associated with the two OPD printings are extremely hard to locate in MNH condition. Full sheets of the OPD printings are collected as such by specialists.
Shortly after the end of World War I, gum rippling was introduced to reduce "curl" in the mint sheets. A gum breaker machine broke the smooth surface of the gum with a pattern of very small, long rectangles. Pay attention to the gumed side of the later issues. They can exist as smooth or ribbed gum. The gum, and the ribbing, was applied before the stamps were printed. Thus some issues can exist with the ribbing rotated 90 degrees (by the printer's mistake). This also indicates that the watermark has been rotated . A notable example is Scott #224 (Michel #241).