Our group just picked up this trinket in the Reign of Winter AP. Just looking to clarify its function.
"Once per day, if used as the focus or divine focus for a summon monster or summon nature’s ally spell, the cauldron automatically conjures 1d3 additional creatures of the same kind from the next lower level list. For example, if a user casts summon monster III to summon 1d3 wolves, he could also summon 1d3 additional wolves (or other creatures from the 2nd-level list)."
Since it only grant "additional creatures of the same kind from the next lower level list" it seems that what's implied, but not outright stated, is that the cauldron has no effect when summoning creatures of the same-level list as that of the spell or when casting the 1st level version of either spell (since there's no lower level list to choose 1d3 creatures from).
I'm also wondering whether the cauldron would work in conjunction with a summoner's Summon Monster spell-like ability seeing as it doesn't require a focus though since, according to the general description of spell-like abilities, it "functions just like a spell. Further, would the Cauldron function in conjunction with appropriate spell-completion or spell-trigger items such as scrolls and wands?
I don't think there's any official ruling on this item, so conjecture is welcome.