It is common in legal contexts to see terms like creditor, debtor, mortgagor, lessor, licensor, guarantor, settlor, and so on. While the meaning of these verb-derived terms seems to be "the one who X-es," (ie credits, debts, mortgages, leases, licenses, guarantees, settles, etc), the typical form for these types of words in English is to add a suffix of -er (ie, player, eater, walker, runner, etc), while -or words, like "author," are comparatively few.
Does the alternative suffix of -or have any significance, and what is the origin of this form of word? It seems like it might derive from either Latin or Law French, but why would such alternative and non-standard spellings continue to remain in use? It is harder to see why they would be retained than it is for certain grammatically idiosyncratic phrases, such as doublets like "cease and desist," or phrases with set word orders remnant from French grammar, like "corporation sole," or "body corporate."
