I can easily believe the Model 21's watertable is about 1/8" longer than the Elsie R; that's consistent with my 21 and the data in Houchins' book. I wouldn't pretend to know the significance of that, however. The 21 is indeed a rugged gun as witnessed by the factory advertising showing the gun being shot with the locking bolt removed and the barrels held down on the frame only by hand. They also shot a 21 with 2000 proof loads without any measurable damage while all the other American and English guns failed with just a fraction of as many proof loads. Also not sure what that really means other than the gun was over-designed to begin with. My 21 has had good care and no abuse during the three decades I have owned it, but I already told you about the repairs to it.
I've never heard of the Higgins 100, but will pay more attention now that you've enlightened me.
As far as other American doubles with long receiver bars go, I can't say. But of the examples I own or have access to, the M21 is the longest.