The detachable action is how the trigger, hammer and block are integral with the bottom metal/trigger guard.
It all pivots out as one unit once the lever pivot screw is removed.
Page 96 in the same book shows a good photo of that.

As discussed above, your bullet will be right at .450” to fit the chamber. That is pretty common on Gibbs rifles and others of the era I suppose - for example, my 461 will not allow a bullet larger than .461”.
Or, perhaps, measure a fired case and go for the bullet that is a good slip fit into the case. I’ve done this with a couple rifles and, as they are single shots, the somewhat loose fit is no worry. Makes reloading a breeze too since no dies are required!
From the photo of the caliper above I assume that is a groove of .459”, correct?
What that means generally is that your best accuracy will be in using real black powder and a cast greased or paper patched bullet on the softer side to “bump up” to the rifling.
I’m sure with your experience you already know all this but it doesn’t hurt going over it again for newcomers to get their heads around how these great rifles work.