In Britain, or England and the British Isles, the use of 15k (usually marked 15ct) was utilized for a number of years. From the year 1854 to 1931, you may find gold jewelry testing at or hallmarked with either 15ct or 625.
In the US, most of us are more familiar with 14k, but some countries, including Britain, used 15k instead. After 1931, the standard for 15k changed to 14k. Those pieces test, or are hallmarked 585 or 14.
This means that the gold content is a little higher, and the presence of alloys such as copper or zinc is a little less. Not much, but all things being equal, the value of 15k gold is slightly higher by weight than 14k gold.
So don't be surprised if you see an antique or vintage gold piece of jewelry that was created with 15k gold.