While on the subject of the 5251, I thought that I would share something that I haven't seen mentioned elsewhere, though I seriously doubt that I'm breaking any new ground here.
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As many of us know, keys from other Beamsprings or from a Selectric can be used to customize our layouts. What I have not heard anyone mention is that due to the Beamspring's split mounting arm, the possibility is there of mounting keys off-center and not using the mounting slot at all. While it may immediately seem like this wouldn't work, in practice I've found that it works really well. For example, here's my current setup on my 5251:
I've taken a few keys from a Selectric II, such as the stepped Caps Lock, Return, and the plus/minus keys on the numpad. Since my muscle memory is ANSI, I swapped the LeftShift and >< keys and also put in a closer-to-ANSI Return key.
While it would seem that these keys shouldn't fit in those spots, thanks to the way the mount has a slot in it, one can just use the key's mold-fins to hold it in place instead of the usual mounting hole. Yes, the key will tip if you press hard on the unsupported side, but I have really not had any issues with regular typing force, even bottoming out the keys pretty hard.
The Return key is mounted similarly but straddles two switches. I am using the original ISO Return/Field-Exit switch for the 'fin-mount', and the backwards-L portion of the key fits snugly over the other switch, making it extremely stable. Two keys are activated for one press in this configuration but that's easily fixed either in programming or by removal of one of the switch plates. I chose to just change the programming to ignore the extra switch.
It was very easy to make these modifications, along with minor adjustments in the xwhatsit controller programming, and didn't require any additional disassembly. These changes, along with some key remaps for modifiers, etc. make this keyboard very usable for me as a daily driver, and I can recommend similar changes for those who have trouble using ISO layouts, but would like to make more use of the 5251 or other Beamspring layout.
Hopefully this info is helpful.