Originally titled "Babricated Forge", Apron is a mod for Minecraft Beta 1.7.3 that adds compatibility with the mods of the era (and those that were made by some very short-sighted developers later on) to the Fabric modding environment.
Apron was originally intended to only offer a compatibility layer with API mods of the time. This would have allowed mods to interact with the APIs themselves, and nothing else! Not even the game itself would have been accessible.
Mod Remapping API is an API mod that can help mods that were designed for one set of mappings function under a different set of mappings by translating calls from one mapping set to another at runtime. Such as in our case: Fabric intermediary names!