In the DMG there are items like the Tome of Leadership and InfluenceTome of Leadership and Influence that gives a permanent +2 to charisma and charisma maximum (and other similar Tomes and Manuals for other abilities), but they all have a rarity of very rare, so it's not really appropriate loot for lower level parties.
I was thinking of giving my low level players a homebrew item that gave a permanent +1 to an ability (I'd roll for which one before giving out the item), up to a maximum of 13, and it wouldn't increase the ability maximum. The idea being that players could use it to increase abilities they might need to multiclass or improve a dump stat. Afterwards, like the Tomes and Manuals, I was thinking that this homebrew item loses its magic and goes dormant for a century. So it's basically a one-shot for one character.
What's a reasonable rarity for an item like this, so I can balance my loot tables? Either uncommon or rare seems reasonable, but I'm having trouble deciding between the two. I'm leaning towards uncommon, given the hard limit of how high it can increase the ability. Without the hard limit I'd guess rare.