Just an FYI, even the Old Covenant had the seeds for indulgences, for example in the famous “Purgatory” apologetic verse in 2 Maccabees:
2 Maccabees 12:43-45 He also took up a collection, man by man, to the amount of two thousand drachmas of silver, and sent it to Jerusalem to
provide for a sin offering. In doing this he acted very well and honorably, taking account of the resurrection. For if he were not expecting that those who had fallen would rise again, it would have been superfluous and foolish to pray for the dead. But if he was looking to the splendid reward that is laid up for
those who fall asleep in godliness, it was a holy and pious thought. Therefore he made atonement for the dead,** that they might be delivered from their sin**.However, the OP said he was not looking for “underpinnings” for indulgences, so I’m not sure that qualifies.
Here’s an article giving some history too, including a breakdown of the 2 Corinthians passage mentioned earlier.