Mike, It appears from the picture that you have a conversion either by Sharps or Meacham.  The lockplate appears to be the thicker version which was used on the percussion models.  Turn it upside down and if there is a screw hole that has a plug screw in it, then it is a percussion lock.  Also, cock the hammer and if there is a slot milled in the lock plate, that  too indicates a percussion lock.  Look carefully as some of those slots were filled with a small piece of steel and are hard to see.  I can't see for sure from the picture but it appears to be a 5 digit number, and if it is, then it's too low for a 74' action.  True 1874's started with a serial number in the high 140,000 range.  Old-Win