A 5 sure does look like an S
I haven't used my Mk2S in a while. Just dusted it off and started getting it going again. Saw that fancy new firmware 4.1.0 announcement and got all excited. I immediately grabbed it and flashed it with avrdude from the command line. Self test fails with a wiring error in the left hotend fan. Because.... the Mk2S didn't come with a fan that has an RPM sensor and I'm an idiot and flashed the Mk2.5 firmware.
So, uh, a 5 really does look like an S. I almost wish Prusa followed dumb American marketing norms and had called the updated printer the "Mk2.five X Maxx Platinum Edition" or something. But hey, that 4.1.0 firmware does actually run. Sort of.
Don't make the same mistake I did.
(You can trivially flash back to the proper 3.1.0 firmware so nothing is lost except some dignity.)