All MC28's use "1" of part number 31420-KV3-680 as the power valve servo motor. So thats pretty easy to understand.

Now when it comes to the MC21, it does indeed look pretty confusing.
There are 2 different part numbers: 31420-KV3-831 and 31420-KV3-680.
31420-KV3-831 comes in type blank and type J
31420-KV3-680 comes in type blank, type J, and type 3J.

I have no idea what the "type" indicates. Be it blank, J or 3J. I would guess it is a subtle difference due to slight design changes over time to improve cost or performance or ease of manufacture. So I would just tend to ignore the "type" categorisation. Maybe this is silly? Someone please correct me if I am wrong.

It appears that the MC28 servo (31420-KV3-680) does indeed work on all models of MC21 as there is a "1" shown at the intersection of that part number and all models of MC21. (disregarding "type")

Please note that I am just making all this up from looking at the Parts Manual and have not physically tried this.
I hope that dilutes the confusion for you a bit?
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