There are something like 1,000 named varieties of avocado. Big, small, green, black, purple, round, pear-shaped, winter, summer, fall harvest, anise smelling leaves, all kinds of distinguishing features. A homeowner once called to ask about the ‘San Marcos' variety of avocado and we viewed images of this tree and fruit and finally figures out it was a ‘Bacon' that was planted on San Marcos Pass and had adopted the new name because they didn't know what to call the avocado tree in the backyard. So there are a lot of trees that are misnamed for known varieties.
If you want to find out the name of an unknown tree in your backyard, there is a convenient online source of information at Avocado Information at UC Riverside. There is an online list with photos of avocado varieties at:
And a variety database you can use to search by name at:
There's also a list of unreleased varieties at:
One of the best sources of variety information is the CA Avocado Society Yearbook where most varieties were listed for registration. Some of the descriptions are online, but in many cases it's necessary to go to the original paperback version
Yearbooks can be found at many UCCE offices in Southern California, UC Riverside and Davis libraries, many Southern CA public libraries and from interlibrary loan.