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So a lot is made online about the variations of JMP and JCM 800 2203's and 2204's over different years. I've owned enough to know that they definitely vary amp to amp.
Its not always possible to hear those differences in a typical usage scenario so hopefully this is helpful with what differences to expect. All 3 amps recently serviced and biased etc by Marshall (big up Dave).
1977 JMP 2203
1979 JMP 2204
1989 JCM 800 2204 (Horizontal Input)
2204's have less negative feedback than 2203's. This makes them a bit looser and brighter. The horizontal input 2204's have higher voltages in the preamp and a different filter cap arrangement for powering the preamp. This gives them a stiffer feel/less squish than lower voltages.
In this example, I used a Maxon OD9 to boost the front end because this particular song called for a bit more gain. I dialled the amps in by ear, probably could have got closer if I spend longer going back and forth, but really, who gives a shit?
Hope this is useful, it was quite fun to try and learn about these amps and each of their quirks. I feel like the 2203 is a bit punchier and bigger sounding, and the vertical input 2204 is a hair more squishy but the differences are pretty much bugger all and not really worth worrying about. Very easy to mod the horizontal 2204's to vertical spec.
