at least 7 bowls, 2 series of mixers, multiple splash rings: extreme chaos
I went looking for a stainless steel bowl for my Bosch Universal mixer. Oh boy. That wasn’t fun. I quickly got a headache. There are at least 6 Bosch bowls spanning two Bosch Universal classifications and no website seemed to agree with any other. Here’s what I learned.
First some history and definitions:
The original Bosch bowl was a flat-bottomed, dough-only bowl that had a three-pronged dough hook that mounted on the bottom of the bowl. This bowl started out with the first Universal mixer in 1951. Some time later Bosch introduced the white, all-purpose, donut-shaped bowl with the center post. I have currently narrowed this down to sometime between 1961 and 1970. I’ll refer to Universal mixers prior to 2007 as Classic Universals. I am lumping together everything from the original 1950’s mixers, to the 2007 redesign into this category – UM3, MUM6, Comfort Plus, the whole lot. The post-2007 mixer is the Universal Plus and will be referred to as such. Note: the links below will take you to pages on this site with more in-depth coverage of each bowl.
The current line-up: 
Bosch makes the MUZ6ER1 stainless steel bowl. This bowl is the original-style dough-only bowl that has shipped with Bosch Universals since their inception. This bowl is the flat-bottomed bowl that has the dough hook mounted to the bottom. This bowl will fit on all Classic Universals as-is. It does not have the built-in locking mechanism on Universal Plus bowls and is unstable on the Universal Plus. To fix this issue people are using rubber bumpers to keep it stable. There have been at least two versions. The current version has 3 locking pins on the inside of the bowl and use the newer splash-ring that fits inside the rim of the bowl. The ER1 has an approximate capacity of 6 quarts by volume and is rated at 14 pounds of dough. 
Bosch also makes the MUZ6ER2 stainless steel bowl. This is the all-purpose stainless bowl designed for the Universal Plus. It is specific to the Universal Plus and will not fit the Classic Universals. This bowl uses the standard French whisks, batter whisks, cookie paddles and dough hook. This bowl has 4 locking pins and requires a 4-pin splash ring. The ER2 has a removable center post for easy clean-up. The ER2 has an approximate capacity of 6 1/2 quarts or 15 pounds of dough. 
L’Equip makes two stainless bowls for Bosch Universal mixers. MUZ6SB3 and MUZ6SB4. These bowls are identical except for the number of splash-ring locking pins. These bowls fit both Classic Universal mixers and Universal Plus mixers. They both have removable drive shafts for easy cleaning. While you can use the SB4 with a Classic Universal and an SB3 with a Universal Plus, you will wind up with mis-matched splash-rings. If this isn’t a concern for you, feel free to mix n match! These bowls are able to hold approximately 5 1/2 quarts or 12 pounds dough.
Bosch makes two all-purpose plastic bowls designed for both kneading and mixing. These are the MUZ6KR4 and MUZ6KR4UC. The MUZ6KR4 has three splash-ring locking pins and uses a splash-ring that fits inside the bowl. This is the bowl that I believe was introduced in 1984 with the MUM6 series of mixers. The KR4 has an approximate capacity of 5 1/2 quarts or 12 pounds of dough. The MUZ6KR4UC bowl is the newest design and is specific to the Universal Plus. It has 4 locking pins and a removable drive shaft for easy cleaning. The UC is rated at approximately 6 1/2 quarts and can knead 15 pounds of dough.
That brought me up to 6 bowls; I thought my research was finished. I had forgotten about the slicer-shredder bowl. I knew that there was a set of whisks for the slicer-shredder bowl – I had even used them. Then L’Equip dropped a bit of a bombshell into the mix with the release of a mini dough hook for the slicer-shredder bowl.
