When was the Levicorpus spell invented?

Question

In Chapter Twelve of Half-Blood Prince Harry discovers the Levicorpus spell in the prince’s potions book:

Sitting up in bed, Harry turned the book sideways so as to examine more closely the scribbled instructions for a spell that seemed to have caused the Prince some trouble. There were many crossings-out and alterations, but finally, crammed into a corner of the page, the scribble:

Levicorpus (nvbl)

This book is the sixth year potions textbook, which would imply that Snape (the prince) invented the spell in his sixth year at Hogwarts.

However, in Chapter Twenty-Eight of Order of the Phoenix Harry sees Snape’s memory in which James used the spell on Snape:

But too late; Snape had directed his wand straight at James; there was a flash of light and a gash appeared on the side of James’s face, spattering his robes with blood. James whirled about; a second flash of light later, Snape was hanging upside down in the air, his robes falling over his head to reveal skinny, pallid legs and a pair of graying underpants.

In Chapter Sixteen of Half-Blood Prince Harry discusses this with Lupin, who confirms that the spell was already popular by then and might have been around even earlier:

“It’s nothing to do with me!” said Harry indignantly. “The Half-Blood Prince is someone who used to go to Hogwarts, I’ve got his old Potions book. He wrote spells all over it, spells he invented. One of them was Levicorpus —”

“Oh, that one had a great vogue during my time at Hogwarts,” said Lupin reminiscently. “There were a few months in my fifth year when you couldn’t move for being hoisted into the air by your ankle.”

“My dad used it,” said Harry. “I saw him in the Pensieve, he used it on Snape.”

He tried to sound casual, as though this was a throwaway comment of no real importance, but he was not sure he had achieved the right effect; Lupin’s smile was a little too understanding.

“Yes,” he said, “but he wasn’t the only one. As I say, it was very popular….You know how these spells come and go….”

“But it sounds like it was invented while you were at school,” Harry persisted.

“Not necessarily,” said Lupin. “Jinxes go in and out of fashion like everything else.”

Given that Snape’s memory was from his fifth year, and the spell had already spread throughout the school by that point, the spell must have been invented prior to Snape’s sixth year. Why, then, would the spell be in his sixth year textbook?

Answer 1

Snape probably had the book in his possession long before his sixth year at Hogwarts.

We know that Snape invented the spell when he had the book, and we know that he invented the spell during or before his fifth year at Hogwarts. Therefore- Snape owned the book before his sixth year.

Why would Snape own the book if Snape isn't on the book's advanced level? Well, that's the thing- Snape is on the book's level. It makes so much sense that Snape, who is so ambitious and advanced in potions, wouldn't wait until his sixth year to learn N.E.W.T. level potions, but would start learning those potions in his fifth year, or even before that.

Answer 2

Severus may already have had this book before his sixth year. In the chapter "A Very Frosty Christmas" there is a mention that it was published 50 years ago, long before Severus even lived; so it may have been a book that was in the Snape family's possession before Severus was even born, so Severus could have written things into it at any time.

Answer 3

It is not absolutely clear as to when the spell was exactly created. Although the spell was written in Snape's book which supposedly started to belong to him in his sixth year, it is possible that he possessed the book since a long time ago than that. He might have created the spell anywhere from being in early days to the fifth year of Hogwarts supposing he recorded it in written form only in a sixth year textbook.

He must have thought of being in need of such dangerous spells as Levicorpus and Sectumsempra besides others since the Marauders had started to bullying and antagonising him from his very first encounter with them in the first year.

From Harry Potter fandom

On the way to Hogwarts for his first year, Severus sat with Lily Evans on the train. While on the train they met James Potter and Sirius Black. This encounter between the three of them revolved around a disagreement regarding what Hogwarts house was the best. This hostile first encounter would set the tone for the antagonism between the three of them for the rest of his life.