There you go, Rick. Dom's drawing explains the way it works perfectly. On a seacoast artillery piece, the short, wheel- bearing axle which protrudes from the larger axle which goes through the carriage cheeks, is just a little bit smaller than the lengthy, cheek axle or hub as he calls it. The photo below shows what an eccentric axle used on an 1859, Iron, Seacoast, Center-Pintle, Barbette, Carriage looks like before the ends are turned off, two scale dimples, only .040" apart (1/6th of .240" on the original axles) are added and the slots are cut for the keys that holds the wheels on. Then the hex-shaped end cuts are made to fit the iron, hand-spike, wrench combination.
As far as the compression brake goes, those photos and drawing thoughtfully provided by Soot pretty much show how that works. Even the wooden shoe is visible just above the bottom bar on the clamp hardware. It binds the securely bolted, rail extension piece to the sliding upper carriage transom.
Tracy & Mike
The left hand axle is in the position to have it's wheels engaged with the Chassis rails, ready for the carriage's 'return to battery'.
The position of the axle on the right is correct for firing the gun, the wheels are retracted up off of the rails and the entire bottom of the left and right cheeks bear on the Chassis rails, in position so maximum friction is caused during firing which stops the carriage's rearward movement within the allotted distance.
P.S. Double D., sure looks like a 'Crank' to me. And, you are getting this observation from a 'cranky eccentric', who should know a 'crank' when he sees one! Tracy