Hi, Beachsimmo70, Welcome here,
Both 1/2 cases were changed in 54 when K became KH. Right and left from the start to slightly change bolt locations around cylinder mouths due to longer KH stroke, then another left one in mid 54 when the trap door arrived.
Check your belly numbers on your 56 set, if they match, 556-xxxx, same xxxx serial # on both sides, you're OK, it's a matched set.
If the right belly is 552 or 553, it won't match the left one. If it's 554 or later you're also OK but will need to check all the bores/faces alignements, crank, mainshaft, cylinder decks and adjust accordingly... KH trap door is not a separate item like on the later Sportster, it was matched to each left case then lined bored so be careful there as well.
Only exception is if a "D" is stamped near the engine serial #, this indicates a replacement set of cases that has been moded by the factory to fit the Sportster trap door, 4 bolts and straight dowels rather than 6 and conical dowels on the KH.