Coy Wolf, the owner of the above original, explained this to me. That the Gov't had given Sharps a $3 limit on the conversions. Supposedly then those conversions using barrel liners, or had damaged barrels replaced, then kept their original hammers, modified, to lower costs. Those conversions that kept their original unaltered barrels got the purpose built hammers.
Caveat: These were issued frontline 1868-1876, and for secondary used another 10 years or so, and used HARD, so some parts mixing no doubt took place.