Buffalo Bill's Needle Gun

In a continuation of my reports from William Cody's autobiography, and the firearms therein, he used to hunt Buffalo in the late 1860s with a Needle Gun he named Lucretia Borgia because it laid low so many or summat.

He described it as a .50 caliber, and he would ride up on the head of a herd of buffalo so as to turn them, and thusly get the group to circle.  Then he could ride along side dropping bison in a relatively small area rather than picking them off from the rear and dropping the carcasses in a 3 mile long line.  All to provide meat for the Kansas Pacific.

A needle gun, eh?  Interesting.  Now, what KIND of needle gun are we talking about here?  The Dreyse, or Chassepot?  Well, quotes from the book:

I took the harness from Brigham, mounted him bareback and started after the game, being armed with my new buffalo killer which I had named "Lucretia Borgia," an improved breech-loading needle-gun which I had obtained from the Government. 

and:

I was using what was known at the time as a needle-gun, a breech-loading Springfield rifle, caliber .50. This was "Lucretia," the weapon of which I have already told you. Comstock's Henry rifle, though it could fire more rapidly than mine, did not, I felt certain, carry powder and lead enough to equal my weapon in execution. 

Well, what kind of creature IS this that Buffalo Bill has in 1868?  Seems neither fish now fowl.  What had Erskine Allin wrought, if indeed Cody has his facts straight?  My guess is that this rifle is just one of the Allin Conversion guns.  Falling block single shot touches off a .50-70 centerfire cartridge.   "Needle Gun" was just the fashionable name at them for such, maybe.   I don't see Buffalo Bill getting a Chassepot with paper cartridges from the US Army.

{Wait, is that floppy top hatch thing really a falling block type?  That doesn't seem like the proper term for the 'trapdoor' mechanism.}