Part of the key is the calliber; the first Allin conversion was the Model 1865. It was almost all rifle musket except for the trapdoor. It fired a .58 rimfire round that did not require any barrel sleeve.
The next was the Model 1866, in .50 centerfire; it also was a conversion, with a sleeved barrel. Ignoring a couple of cadet rifles, the next was the Model 1868, also .50 CF, which had a newly made true receiver instead of the cut barrel of the previous models, and a newly made unlined barrel, but still used Civil War lockplates and other parts.
Next up was the Model 1870, which had a different and improved receiver, but was otherwise very similar to the Model 1868.
Then came the Model 1873, in .45-70, almost totally new, though the earlier ones had Civil War era trigger guards.
So there was quite a story to the development of the Allin action.
Jim