Complex sentence: Jeeves's first appearance was in "Extricating Young Gussie", which was published in 1915 the US; multiple Wodehouse reference books say that Jeeves first appeared in 1916, possibly because that is when he appeared in both the US and the UK.
Split into simpler sentences: Jeeves's first appearance was in "Extricating Young Gussie", which was published in 1915 the US. However, multiple Wodehouse reference books say that Jeeves first appeared in 1916, possibly because that is when he first appeared in both the US and the UK.