Here are some things you start to notice about the classic Tamagotchi shells after going through hundreds of photos for them for the Wiki.
The Gen 1 had a series of shells with the pattern of a clock face. Three shells were produced in Japan, and two in Europe. None came to America.
This was a similar series with a scattered numbers pattern. One was produced in Japan, and the remaining four in Europe. Again, none of these came to America.
These two Gen 1 shells uses the same print pattern, but with different colors. Left is Japan, right is America.
America got a unique series of two-tone shells; the bottom halves of these shells were painted a different color from the top half. Japan would later get the black/orange shell as a JAL special edition, with the English programming.
During Gen 2, many English shell designs had the same colors as the Japanese ones, but simplified with the English logo.
There were two Golden Tamagotchis. The one on the left is a Japanese gen 1. It and a matching silver gen 1 were sold during New Year’s Eve 1996 as part of a market test. The one on the right is an English gen 2, and was sold exclusively at US Toys R Us stores during the 1997 holiday season.