The first trapdoors were the M1865 "First Model Allen Conversion" in .58 Rimfire, still with 40 inch long barrels.
A farely large number were made, since it was a cheep, easy conversion. But the Army did not like them and few were ever issued. However many were sold to the Union Pacific, the largest number of their spent cases can be found along the original trans-continental railroad right of way.
I do not beleve though that any trapdoors were issued before April 1865.
The M1866 was a much more radical conversion. Requiring not only adding the trapdoor action block at the breech but also boring out and resleeving the barrel to take it down to .50 cal for the .50/70 centerfire cartridge and then shortening the barrel by 10 inches at the front and resighting.
Most Army units in the west went streight from the rifled muzzle loader to the M1866.