The relevant versioning history goes as follows:
Although not legitimately obtainable in-game, translation strings exist for enchantment levels 6-10, with roman numerals used as expected. These can presumably be seen with hacked items through NBTEdit or similar.
MCP 4.5 for b1.9-p5 is not well-documented. Its existence was never mentioned on Twitter by Searge, it is not listed on the "temporary" downloads page (which only lists MCP for Minecraft 1.7.10+), and generally cannot be found elsewhere besides the Minecraft Wiki.
Enchantment child classes are given names by MCP which are close to, but not identical to, the names of the enchantments. For example, Silk Touch's class is named "Untouching". This seems to come from the translation strings for the enchantments. It also may be to ensure that enchantments sharing the same class do not get confusing (for example, Looting and Fortune share the EnchantmentLootBonus class).
Some websites which exist from the time give incorrect values, especially for maximum enchantment levels. One site, for example, had Sharpness and Efficiency as both going to only 4 rather than 5. This may be due to the difficulty of obtaining high-level enchantments, although /xp on a server makes the process substantially easier.
There is a bug where collecting a large amount of xp at once will increase levels only at the rate for the first level. For example, it takes 7xp to get to level 1, but 14 to get to level 2. If you had 0 xp and obtained 14, that would give you two levels, as it is enough for two times the next level. This only applies when the xp is applied all at once, which likely could not happen except when using the /xp command.
Enchantments have a method for the minimum valid level. Since all enchantments start at level 1, this method is never overridden and always set to 1.