Thank you gracielove.
Located this 2015 series of two articles on
LostDogsofAmerica.
"The Microchip Maze -- Buyer Beware (Part 1- The 900 Chips)"
"The Microchip Maze - Searching the Databases (Part 2)"
The first article stresses the importance of chips that support coding by the International Committee for Animal Recording (ICAR). At least as of the article date, author calls out the "big 5 microchip companies" saying their chips have the right coding.[1] The five companies with the coding are:
*PetLink
*Home Again
*AKC
*AVID
*24 Petwatch
According to the author, there are many, many smaller companies using cheaper chips that share a "900" code. (Six companies/brands are mentioned by name.) Because the companies are sharing the chip code, it is apparently a more complicated process ("quagmire") to wade through all the steps necessary to identify the pups owner. Article suggests this likely prevents many pups from being reunited.
Second article is focused on the registries. Mentions a tool created by the American Animal Hospital Association--"AAHA Universal Pet Microchip Lookup Tool." This application "enables veterinarians, shelters, animal control facilities, pet owners or the public to search various registries and identify those registries on which a particular microchip is registered."
At the time the second article was written/edited, only four of the big five microchip companies were participating in AAHA Universal Pet Microchip Lookup. (AVID, which supplied my late Katy's chip, was not participating.) Current list of participating brands is
here.